Chao Zheng

College of Charleston, Charleston, SC, United States

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Publications (44)85.45 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Variable temperature (−55 to −155 °C) studies of the infrared spectra (400–3500 cm−1) of ethylisocyanate, CH3CH2NCO, dissolved in liquid xenon and krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate two conformers in the fluid states which are the cis and trans forms with a large proportion of molecules in the gas phase at ambient temperature in the excited states of the NCO torsional mode which has a very low barrier to conformational interchange. Variable temperature (−110 to −155 °C) studies of krypton solutions were carried out and by using two conformer pairs, an enthalpy difference of 100 ± 4 cm−1 (1.20 ± 0.05 kJ/mol) was obtained with the cis conformer the more stable form. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with full electron correlation using a variety of basis sets up to 6-311+G(2df,2pd) and cc-PVQZ. With the basis sets 6-311+G(2d,2p) and larger, the barrier at the cis position ranged from a low value of 11 cm−1 to a high value of 31 cm−1 with a value of 19 cm−1 from the largest basis set of cc-PVQZ. Thus, the gauche well is probably so shallow that it does not contain a bound vibrational state. This results in the cis conformer as the most stable form which is consistent with the experimental rotational and vibrational data. The predicted energy difference from these calculations between the cis conformer and the transitional-state skew form is ∼100 cm−1 which is consistent with the assigned microwave lines for four excited states of the NCO torsion. Density functional theory calculation by the B3LYP method with many of the same basis sets provided little information. By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(CN) = 1.211(5), r(CO) = 1.167(5), r(CN) = 1.448(5), r(CC) = 1.516(5) Å for the distances and angles of ∠CCN = 112.6(5), ∠CNC = 137.5(5), ∠NCO = 172.9(5)°. The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.
    Journal of Molecular Structure 05/2010; · 1.40 Impact Factor
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    ABSTRACT: Variable temperature (−55 to −131 °C) studies of the infrared spectra (4000–50 cm−1) of 2-fluoroethylamine, FCH2CH2NH2 dissolved in liquid xenon and the far infrared in liquid krypton have been carried out. From these data all five possible conformers have been identified and their relative stabilities obtained. The enthalpy differences have been determined among the most stable Gg′ conformer and the second stable conformer, Gt, to be 62 ± 8 cm−1 (0.74 ± 0.10 kJ/mol), the third stable conformer, Tg, to be 262 ± 26 cm−1 (3.14 ± 0.3 kJ/mol), the fourth most stable conformer, Tt, to be 289 ± 45 cm−1 (3.46 ± 0.5 kJ/mol) and the fifth most stable conformer, Gg to be 520 ± 50 cm−1 (6.24 ± 0.6 kJ/mol). The percentage of each conformer at ambient temperature is estimated to be with Gg′ (42 ± 4%), Gt (32 ± 1%), Tg (13 ± 1%), Tt (5 ± 1%) and Gg (3 ± 1%). The first indicator is the NCCF dihedral angle (G = gauche or T = trans) and the second one (g = gauche or t = trans) is the relative position of the lone pair of electrons on the nitrogen atom with respect to the fluorine atom. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the two most stable conformers. The determined heavy atom structural parameters for the Gg′[Gt] conformer are: the distances (Å) C1–C2 = 1.509(3) [1.516(3)], C2–N3 = 1.466(3)[1.461(3)], C1–F4 = 1.400(3)[1.398(3)] and angles in degrees ∠N3C2C1 = 109.8(5) [115.5(5)], ∠F4C1C2 = 109.2(5)[109.3(5)], ∠H10N3H9 = 107.1(5) [107.1(5)](°) and τF4C1C2N3 = 65.3(5) [60.9(5)]. Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for all five conformers. The results are discussed and compared to the corresponding properties of some similar molecules.
    Journal of Molecular Structure 01/2010; 968:36-47. · 1.40 Impact Factor
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    ABSTRACT: The Raman (3300-10 cm(-1)) and infrared (3300-40 cm(-1)) spectra of gaseous and solid vinyl silyl fluoride, CH(2)=CHSiH(2)F, have been recorded. Raman spectrum of the liquid has also been recorded and depolarization values obtained. Variable-temperature studies of the infrared spectra of the sample dissolved in liquid krypton (-110 to -150 degrees C) and liquid xenon (-60 to -100 degrees C) have been carried out. From these studies, the enthalpy difference has been determined to be 76 +/- 7 cm(-1) (0.91 +/- 0.08 kJ/mol) from the krypton solutions and 69 +/- 7 cm(-1) (0.82 +/- 0.08 kJ/mol) from the xenon solutions, with the gauche conformer the more stable form. From the far-infrared spectrum of the gas, the asymmetric torsional fundamentals for the cis and gauche conformers have been observed at 102.34 and 86.56 cm(-1), respectively, with each having several "hot bands" falling to lower frequencies. From these frequencies along with the experimentally determined conformational enthalpy difference, as well as the gauche skeletal dihedral angle, the potential function governing the conformational interchange has been determined with the following Fourier cosine potential coefficients: V(1) = -80 +/- 11, V(2) = -42 +/- 15, V(3) = 622 +/- 5, V(4) = 34 +/- 5, and V(6) = -31 +/- 2 cm(-1). The gauche-to-cis and gauche-to-gauche barriers are 664 cm(-1) (7.94 kJ/mol) and 608 cm(-1) (7.27 kJ/mol), respectively. Complete vibrational assignments are provided for both conformers. In addition, equilibrium geometries and electronic energies have been determined for both rotamers from ab initio calculations using restricted Hartree-Fock and Møller-Plesset perturbation method to the second order (MP2), as well as density functional theory by the B3LYP methods, employing a number of basis sets up to 6-311+G(2df,2pd). All levels of calculation predict the gauche conformer to be the more stable form. By systematically adjusting the ab initio predicted structural values to fit the previously reported microwave rotational constants, adjusted r(0) parameters have been obtained for both conformers. These values are compared to those for the corresponding chloride and methyl compounds. The spectroscopic and theoretical results are discussed and compared to the corresponding quantities for some similar molecules.
    The Journal of Physical Chemistry A 02/2009; 113(8):1653-62. · 2.77 Impact Factor
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    ABSTRACT: Variable temperature (−55 to −145 °C) studies of the infrared spectra (4000–50 cm−1) of allyl alcohol (3-hydroxypropene), CH2CHCH2OH, dissolved in liquid krypton and/or liquid xenon have been carried out. The infrared spectra of the gas and solid have also been recorded. From these data four of the five possible stable conformers have been identified and their relative stabilities determined. The order of the conformers stabilities has been obtained experimentally where the first indicator is for the relative position of the OH group (C = cis or G = gauche) to the double bond (rotation around the CC bond) and the second one (t = trans, g = gauche, g′ = gauche′) is the relative position of the hydroxyl rotor, i.e. rotation around the CO bond. The enthalpy differences have been determined between the most stable Gg conformer and the second most stable rotamer, Cg, to be 135 ± 14 cm−1 (1.62 ± 0.1 kJ/mol), the third most stable conformer Ct, 260 ± 46 cm−1 (3.11 ± 0.6 kJ/mol), and the fourth most stable conformer Gt 337 ± 75 cm−1 (4.03 ± 0.9 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Gg > Cg > Ct > Gt > Gg′. No evidence was obtained for the fifth conformer Gg′ which is predicted by most of the ab initio calculations to be less stable by an enthalpy difference more than 500 cm−1 than the Gg form. The percentage of each conformer at ambient temperature is estimated to be Gg(54 ± 2%), Cg(28 ± 2%), Ct(8 ± 2%) and Gt(11 ± 3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from ab initio MP2(full)/6-31G(d) calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Gg and Cg conformers along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for these two conformers. The determined heavy atom structural parameters for the Gg [Cg] conformer are: the distances(Å) CC = 1.343(3)[1.340(5)], CC = 1.499(3)[1.504(5)], CO = 1.428(3)[1.419(5)] and angles(o) ∠CCC = 122.8(5)[124.7(5)], ∠ CCO = 112.1(5)[114.9(5)] and ∠ CCCO = 122.7(10)[5.63(5)]. Several of the CH distances are significantly different from the values previously reported from the earlier microwave data. The results are discussed and compared to the corresponding properties of some similar molecules.
    Journal of Molecular Structure 01/2009; · 1.40 Impact Factor
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    ABSTRACT: Raman spectra of liquid cyclohexane, C6H12, and deuterated cyclohexane, C6D12, were recorded with both parallel and perpendicular polarizations. The observed vibrational wavenumbers, depolarization ratios, and their intensities were measured and compared with the corresponding predicted values as well as the experimental values previously reported. The conformational energetics were obtained with the Møller–Plesset perturbation method to the second order [MP2(full)] as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. The average ab initio predicted difference in energy between the more stable chair form (D3d) and the less stable twisted-boat form (D2) is 2213 cm−1 (26.47 kJ/mol), with a similar value of 2223 cm−1 (26.59 kJ/mol) from the density function theory calculations. By using two dihedral angles as variables, we calculated the chair–boat interconversion pathway for cyclohexane at the MP2(full)/6-31G(d) level. The harmonic force constants, Raman intensities, depolarization values, and the potential energy distribution were predicted from both MP2(full) and B3LYP calculations with the 6-31G(d) basis set and compared with the experimental values for the chair form when available. The ‘adjusted’ r0 structural parameters were obtained from MP2/6-311 + G(d,p) calculations and previously reported microwave rotational constants of five isotopomers of cyclohexane: i.e. 1,1-d2, 13C-1,1-d2, 1,1,2,2,3,3-d6, and d1 (equatorial and axial). The determined distances in Å are: r(CC) = 1.536(3), r(CH)ax = 1.098(1); r(CH)eq = 1.095(1); and the angles in degrees: ∠CCHax = 108.8(3); ∠CCHeq = 110.2(3); ∠CCC = 111.1(3); and ∠HCH = 107.6(3) with dihedral angle ∠CCCC = 55.7(3). These values are compared with those previously reported and it is found that the difference in the r0 distances (0.003 Å) between the two CH values is much smaller than the difference (0.008 Å) previously reported for the rs values. Copyright © 2008 John Wiley & Sons, Ltd.
    Journal of Raman Spectroscopy 09/2008; 40(2):197 - 204. · 2.68 Impact Factor
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    ABSTRACT: Variable temperature (-115 to -155 degrees C) studies of the infrared spectra (3200-400 cm-1) of 4-fluoro-1-butene, CH2=CHCH2CH2F, dissolved in liquid krypton have been carried out. The infrared spectra of the gas and solid as well as the Raman spectra of the gas, liquid, and solid have also been recorded from 3200 to 100 cm-1. From these data, an enthalpy difference of 72 +/- 5 cm-1 (0.86 +/- 0.06 kJ x mol-1) has been determined between the most stable skew-gauche II conformer (the first designation refers to the position of the CH2F group relative to the double bond, and the second designation refers to the relative positions of the fluorine atom to the C-C(=C) bond) and the second most stable skew-trans form. The third most stable conformer is the skew-gauche I with an enthalpy difference of 100 +/- 7 cm-1 (1.20 +/- 0.08 kJ x mol-1) to the most stable form. Larger enthalpy values of 251 +/- 12 cm-1 (3.00 +/- 0.14 kJ x mol-1) and 268 +/- 17 cm-1 (3.21 +/- 0.20 kJ x mol-1) were obtained for the cis-trans and cis-gauche conformers, respectively. From these data and the relative statistical weights of one for the cis-trans conformer and two for all other forms, the following conformer percentages are calculated at 298 K: 36.4 +/- 0.9% skew-gauche II, 25.7 +/- 0.1% skew-trans, 22.5 +/- 0.2% skew-gauche I, 10.0 +/- 0.6% cis-gauche, and 5.4 +/- 0.2% cis-trans. The potential surface describing the conformational interchange has been analyzed and the corresponding two-dimensional Fourier coefficients were obtained. Nearly complete vibrational assignments for the three most stable conformers are proposed and some fundamentals for the cis-trans and the cis-gauche conformers have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared, and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable. The adjusted r0 structural parameters have been determined by combining the ab initio predicted parameters with previously reported rotational constants from the microwave data. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.
    The Journal of Physical Chemistry A 04/2008; 112(11):2268-81. · 2.77 Impact Factor
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    ABSTRACT: Infrared spectra (4000–50 cm−1) have been recorded for three isotopomers of aminodifluorophosphine, H2NPF2, H215NPF2 and D2NPF2, of the gases. The Raman spectra of all three molecules were recorded of the liquids. To support the vibrational assignment MP2(full) ab initio calculations with the 6-31G(d) basis set were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. Additional ab initio calculations employing a variety of basis sets with and without diffuse functions have been used to predict the conformational stability, structural parameters and centrifugal distortion constants. These predictions are compared to previously reported experimental values when available. The adjusted r0 structural parameters have been obtained by systematically fitting the MP2(full)/6-311+G(d) predicted values with the previously reported rotational constants for five isotopomers obtained from the microwave study. The difference in the two NH distances is 0.003 Å which is much smaller than the value of 0.021 Å previously reported. The bonding around the nitrogen atom is effectively planar which is consistent with the previously reported structural information from the microwave study but differs from the reported slightly pyramidal bonding obtained in the electron diffraction investigation. The adjusted r0 heavy atom distances and angles are: r(PF) = 1.587(3); r(NP) = 1.649(3) Å; ∠FPF = 94.7(5); ∠NPF = 100.6(5)°. Vibrational assignments are given for H2NPF2, H215NPF2 and D2NPF2 which are supported by ab initio MP2/6-31G(d) calculations. These results are compared to the corresponding quantities of some similar molecules.
    Journal of Molecular Structure 01/2008; · 1.40 Impact Factor
  • James R Durig, Chao Zheng
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    ABSTRACT: Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 12/2007; 68(3):783-95. · 1.98 Impact Factor
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    ABSTRACT: The infrared spectra (3500–400 cm−1) of CH3NCS in gas, xenon solution (−60 to −100 °C) and solid phases have been recorded. The effect of free internal rotation of the methyl group is observed on the CH3 antisymmetric stretch and deformation bands from which zeta values have been obtained. The band centers for the degenerate CH3 vibrations have also been determined from the low-temperature spectra in liquid xenon and their values are significantly different from the ones in the gas. By combining previously reported rotational constants for CH3NCS, SiH3NCS and GeH3NCS with the ab initio MP2(full)/6-311+G(d,p) predicted parameters, adjusted r0 parameters have been obtained. The structural parameters for CH3NCS (Cs) are: r(CN) = 1.441(5), r(NC) = 1.206(5), r(CS) = 1.586(5), r(CHs) = 1.090(2) and r(CHa) = 1.093(2) Å with ∠CNC of 143.9(3) and ∠NCS of 175.5(2)°; for SiH3NCS (C3v): r(SiN) = 1.704(5), r(NC) = 1.197(5), r(CS) = 1.567(5), r(SiH) = 1.476(3) Å with ∠NSiH = 108.5(3)°; for GeH3NCS (Cs): r(GeN) = 1.843(5), r(NC) = 1.209(5), r(CS) = 1.576(5), r(GeHs) = 1.519(3) and r(GeHa) = 1.517(3) Å with ∠GeNC = 149.9(5) and ∠NCS = 176.5(2)°. These parameters are significantly different from the values previously reported for all three molecules. Ab initio calculations by the Møller-Plesset perturbation method to the second order MP2(full) and density functional theory calculations by the B3LYP method utilizing a variety of basis sets have been carried out for all three molecules to predict barriers to rotation and linearity as well as force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios and centrifugal distortion constants which are compared to the experimental quantities. Some significant changes have been made for the assignments of some of the fundamentals for CH3NCS which has been clearly shown to have Cs symmetry in the ground vibrational state as well as for GeH3NCS with Cs symmetry. The results are discussed and compared to corresponding quantities of some similar molecules.
    Journal of Molecular Structure. 01/2007; 829:88-110.
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    ABSTRACT: Variable temperature (−60 to −100 °C) studies of the infrared spectra (3200–400 cm−1) of isopropylisothiocyanate, (CH3)2CHNCS, dissolved in liquefied xenon, have been carried out. Additionally the infrared spectra of the gas and solid have been recorded for both isopropylisothiocyanate and tertiary-butylisothiocyanate, (CH3)3CNCS, from 3200 to 100 cm−1. The analyses of these spectral data for the isopropyl molecule and the Raman spectrum of the liquid indicated one stable conformer (trans) in the annealed solid but in the fluid phases most of the molecules have energies above the barriers of the two predicted bound vibrational states i.e., trans and gauche forms. The MP2(full) ab initio calculations, employing a variety of basis sets with and without diffuse functions, have been used to predict the conformational stabilities with the trans conformer, the most stable form, for isopropylisothiocyanate and the second most stable form is predicted to be either the skew or gauche conformer depending in some cases on whether diffuse functions are used. However, even when the energy values indicate the skew form more stable than the gauche rotamer, one imaginary frequency indicates the skew form is a first-order saddle point. These results should be contrasted with the microwave data where the experimental B + C value was previously interpreted to indicate the skew conformer as the most stable form. For t-butylisothiocyanate the staggered conformer is the more stable form with the eclipsed conformer a transition state with a barrier of ∼50 cm−1 so there is nearly free internal rotation of the NCS moiety. For both molecules, the structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and complete vibrational assignments are proposed. The r0 structural parameters are estimated by combing the MP2(full)/6-311+G(d,p) predicted values for the CH parameters with some adjustments to the heavy atom distances. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.
    Journal of Molecular Structure 01/2007; 839:107-124. · 1.40 Impact Factor
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    ABSTRACT: Variable temperature (-55 to -150 degrees C) studies of the infrared spectra (3200-100 cm(-1)) of cyclopropylmethyl isothiocyanate, c-C(3)H(5)CH(2)NCS, dissolved in liquefied rare gases (Xe and Kr), have been carried out. The infrared spectra of the gas and solid, as well as the Raman spectrum of the liquid, have also been recorded from 3200 to 100 cm(-1). By analyzing six conformer pairs in xenon solutions, a standard enthalpy difference of 228 +/- 23 cm(-1) (2.73 +/- 0.27 kJ.mol(-1)) was obtained with the gauche-cis (the first designation indicates the orientation of the CNCS group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCS group with respect to the bridging C-C bond) rotamer the more stable form, and it is also the only form present in polycrystalline solid. Given statistical weights of 2:1 for the gauche-cis and cis-trans forms (the only stable conformers predicted); the abundance of cis-trans conformer present at ambient temperature is 14 +/- 2%. The potential surface describing the conformational interchange has been analyzed, and the corresponding two-dimensional Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche-cis conformer is predicted to be more stable by 159-302 cm(-1), which is consistent with the experimental results. However, without diffuse functions, the conformational energy differences are nearly zero even with large basis sets. For calculations with density functional theory by the B3LYP method, basis sets without diffuse functions also predict smaller energy differences between the conformers, although not nearly as small as the MP2 results. A complete vibrational assignment for the gauche-cis conformer is proposed, and several fundamentals for the cis-trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from ab initio calculations and compared to the experimental values when applicable; the r(0) structural parameters are also estimated. The energies for the linear CNCS moiety were calculated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.
    The Journal of Physical Chemistry A 08/2006; 110(29):9057-70. · 2.77 Impact Factor
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    ABSTRACT: The microwave spectra of (methylamino)thiophosphoryl difluoride, CH(3)NHP(=S)F(2), and two deuterated species, CH(3)NDP(=S)F(2) and CD(3)NHP(=S)F(2), have been investigated in the region from 26.5 to 39.0 GHz. The rotational constants of the ground vibrational state have been determined and have been shown to be only consistent with the trans conformer (CH(3) group antiperiplanar to the P=S bond) with C(s) symmetry. The a-type R branch transitions have been assigned for the trans conformer for the three isotopomers on the basis of the rigid rotor model. Near-trans and near-cis forms without molecular planes of symmetry are predicted by all ab initio calculations with the near-trans form being more stable. However, the double-well potentials governing the interchange between the two enantiomeric near-trans as well as the two near-cis forms are too shallow to accommodate the zero-point energies of the nu(24) asymmetric torsion. Thus, the trans conformation with C(s) symmetry may be more accurate in explaining the microwave experimental data. The "adjusted" r(0) structural parameters have been obtained by systematically adjusting the ab initio MP2(full)/6-311+G(d,p) structure of the trans conformer with C(s) symmetry to fit the microwave rotational constants. The determined heavy atom distances are r(C-N) = 1.459(5), r(P-N) = 1.621(5), r(P=S) = 1.879(5), and r(P-F) = 1.550(5) A, and the heavy atom angles are angleCNP = 124.7(5) degrees , angleNPS = 118.3(5) degrees , angleNPF = 103.2(5) degrees , angleFPS = 117.0(5) degrees , and angleFPF = 94.6(5) degrees . The adjusted r(0) parameters have also been obtained for aminodifluorophosphine, H(2)NPF(2), with a slightly pyramidal -PNH(2) moiety. The results indicate that the previously reported short distance of 0.981(5) A for the N-H(o)(outer) bond from the microwave study is too short, and the adjusted r(0) value of 1.007(3) A is obtained from the combined data. Adjusted r(0) parameters are also reported for (dimethylamino)difluorophosphine, (CH(3))(2)NPF(2), with C(s) symmetry with the PNC(2) portion of the molecule being planar. The previously reported C-H distances from the electron diffraction study are too long, and the anglePNC(i) and angleC(o)NC(i) angles are also found to be in error. These results provide a reasonable explanation why the microwave and electron diffraction results differ for the structures of these latter two molecules.
    The Journal of Physical Chemistry A 08/2006; 110(26):8037-43. · 2.77 Impact Factor
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    ABSTRACT: Variable temperature (-55 to -145 degrees C) studies of the infrared spectra (3500 to 100 cm(-1)) of ethylamine, CH(3)CH(2)NH(2), dissolved in liquid krypton and/or xenon have been recorded. From these data, the enthalpy differences have been determined to be 54 +/- 4 cm(-1) (0.65 +/- 0.05 kJ/mol), with the trans conformer (methyl group relative to the lone pair of electrons on nitrogen) being the more stable form. It is estimated that there is 61 +/- 1% of the doubly degenerate gauche form present at ambient temperature. The conformational energetics have been calculated with the Møller-Plesset perturbation method to the second order (MP2(full)) and the fourth order (MP4(SDTQ)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. Basis sets with diffuse functions lead to incorrect prediction of the conformational stability. On the basis of the frequencies of the torsional transitions along with the determined experimental enthalpy difference and gauche dihedral angle, the potential function governing conformational interchange has been obtained, and the determined Fourier cosine coefficients are V(1) = -207 +/- 48, V(2) = 320 +/- 67, V(3) = 1072 +/- 25, V(4) = 55 +/- 11, and V(5) = -96 +/- 28 cm(-1), with a trans-to-gauche barrier of 1286 cm(-1), and a gauche-to-gauche barrier of 715 cm(-1). The 3-fold methyl rotational barriers have been determined to be 1241 +/- 4 and 1281 +/- 10 cm(-1) for the gauche and trans conformers, respectively. By utilizing the previously reported microwave rotational constants combined with the structural parameters predicted at the MP2(full)/6-311+ G(d,p) level, adjusted r(0) structural parameters have been obtained. A complete vibrational assignment is given for the trans conformer, which is supported by normal coordinate calculations utilizing scaled force constants from ab initio B3LYP/6-311++G(3df,3pd) calculations. Proposed assignments are also made for the fundamentals of the gauche conformer. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for similar molecules.
    The Journal of Physical Chemistry A 06/2006; 110(17):5674-84. · 2.77 Impact Factor
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    ABSTRACT: The Raman spectrum (4000–200 cm−1) of liquid and infrared spectra (4000–400 cm−1) of gaseous and liquid 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane, c-OSi(CH3)2CH2CH2Si(CH3)2, have been recorded. Ab initio and density functional theory (DFT) calculations with several different basis sets including diffuse functions have been carried out to predict the conformational stabilities with comparisons to disiloxane (H3SiOSiH3)and 2,5-disila-1-oxacyclopentane (c-OSiH2CH2CH2SiH2). These calculations predict the twistedC2 conformer as the most stable form of the ring compounds, with the planar C2v conformer a transition state and the envelope Cs conformer not a stable form. These predicted stabilities are supported by the vibrational data. Additionally, force constants, infrared intensities, Raman activities, depolarization ratios and scaled vibrational wavenumbers have been determined from MP2(full)/6–31G(d) calculations for all three molecules and compared with experimental values when available. Predicted r0 structural parameters have been obtained for dimethyl ether, methyl silyl ether and disiloxane, which are compared with the experimentally determined values. On the basis of these data, estimated r0 parameters are provided for both five-membered rings with the expectation that these values are as accurate as could be measured experimentally in the gas phase. The results are discussed and compared with corresponding quantities for some similar molecules. Copyright © 2006 John Wiley & Sons, Ltd.
    Journal of Raman Spectroscopy 01/2006; 37(1‐3):52 - 67. · 2.68 Impact Factor
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    ABSTRACT: Variable temperature (−55 to −100 °C) studies of the infrared spectra (3200 to 100 cm−1) of cyclopropylmethyl isocyanate, c-C3H5CH2NCO, dissolved in liquefied xenon, have been carried out. The infrared spectra (gas and solid) as well as the Raman spectrum of the liquid have been recorded from 3200 to 100 cm−1. By analyzing six conformer pairs in xenon solutions, an enthalpy difference of 193 ± 19 cm−1 (2.31 ± 0.23 kJ/mol) was obtained with the gauche–cis rotamer (the first designation indicates the orientation of the CNCO group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCO group with respect to the bridging CC bond) the more stable form and the only form present in polycrystalline solid. The abundance of the cis–trans conformer present at ambient temperature is 16 ± 1%. The potential function governing the conformational interchange has been obtained from B3LYP/6-31G(d) calculations and the two-dimensional potential has been obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche–cis conformer is predicted to be more stable by 223 to 269 cm−1, which is consistent with the experimental results. However, without diffuse functions the predicted conformational energy differences are much smaller (77–166 cm−1). Similar diffuse function dependency affects density functional theory calculations by the B3LYP method to a lesser extent. A complete vibrational assignment for the gauche–cis conformer is proposed and several fundamentals for the cis–trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and r0 structural parameters are estimated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.
    Journal of Molecular Structure 01/2006; 800:120-134. · 1.40 Impact Factor
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    ABSTRACT: The Raman spectra (4000–100 cm−1) of liquid and solid and infrared spectra (4000–400 cm−1) of gaseous, liquid and solid cyclohexyl silane, c-C6H11SiH3, have been recorded and assigned. These spectral data indicate the presence of two conformers in the fluid states. Variable temperature (21 to −71 °C) Raman spectra of the liquid were also recorded and by utilizing three conformer pairs, an enthalpy difference of 520±70 cm−1 (6.22±0.84 kJ/mol) was obtained with the chair-equatorial form the more stable conformer. It is calculated that at ambient temperature there is only 7.5±2.4% of the chair-axial form present in the liquid phase. Comparison with the spectra of the polycrystalline solid phase shows that the chair-equatorial conformer is the only form remaining in the solid. MP2 and DFT calculated conformational energy differences are slightly larger but in reasonable agreement with the experimental value. In addition, force constants, infrared intensities, Raman activities, depolarization ratios, scaled vibrational frequencies and potential energy distributions have been calculated from the MP2(full)/6–31G(d) results for both chair forms. These data support the complete vibrational assignment for the chair-equatorial form as well as the assignments for several of the fundamentals of the chair-axial form. By utilizing a series of sum and difference bands on the SiH stretching modes, the barrier to SiH3 internal rotation has been determined to be 684±10 cm−1 (8.18±0.12 kJ/mol) for the chair-equatorial form. Estimated r0 structural parameters have been obtained for both conformers by adjusting MP2(full)/6–311+G(d,p) structural predictions. Three additional twist forms (equatorial, axial and form III) were also predicted to be local minima since all calculated vibrational frequencies are real. However, all three twist forms are much higher in energy (2000–3000 cm−1) than the chair forms. Effects of electronegativity and steric effect on the conformational stability are compared among a series of mono-substituted cyclohexanes by NBO analyses of the donor-acceptor delocalization interactions.
    Journal of Molecular Structure 01/2006; · 1.40 Impact Factor
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    ABSTRACT: Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d), 6-311+G(d,p), and 6-311+G(2d,2p) basis sets have been carried out for the XNCS and XSCN (X=H, F, Cl, Br) molecules. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and structural parameters have been determined and compared to the experimental quantities when available. By combining previously reported rotational constants for HNCS, ClSCN and BrSCN with the ab initio MP2/6-311+G(d,p) predicted parameters, adjusted r0 parameters have been obtained. The structural parameters for isothiocyanic acid are: r(H–N)=0.996(5) Å; r(NC)=1.208(3) Å; r(C=S)=1.568(3) Å; ∠HNC=132.0(5)°; ∠NCS=172.9(5)°. These reported angles have much lower uncertainties than those previously reported from the microwave data alone. The ab initio predicted barrier to molecular linearity of HNCS is 917±44 cm−1 which is in excellent agreement with the experimental value of 900±200 cm−1. The r0 parameters for ClSCN are: r(Cl–S)=2.023(5); r(S–C)=1.694(3); r(CN)=1.156(3) Å; ∠ClSC=99.6(5) and ∠SCN=174.4(5)° and for BrSCN are: r(Br–S)=2.197(5); r(S–C)=1.681(3); r(CN)=1.158(3) Å; ∠BrSC=99.5(5) and ∠SCN=174.7(5)°. Structural parameters are also estimated for HSCN and FSCN. The predicted frequencies for the fundamentals of HNCS and DNCS compare favorably to the experimental values but several of their predicted intensities differ significantly from the observed spectra. The frequencies for the reported bending modes of ClSCN and BrSCN do not agree with the predicted values and further experimental studies are needed for providing confident assignments for these fundamentals.
    Journal of Molecular Structure 01/2006; 784:78-92. · 1.40 Impact Factor
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    ABSTRACT: The Raman (3500–30cm−1) spectra of liquid and solid and the infrared (3500–40cm−1) spectra of gaseous and solid 3-methyl-3-butenenitrile, CH2C(CH3)CH2CN, have been recorded. Both cis and gauche conformers have been identified in the fluid phases but only the cis form remains in the solid. Variable temperature (−55 to −100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 163±16cm−1 (1.20±0.19kJmol−1), with the cis conformer the more stable rotamer. It is estimated that there is 48±2% of the gauche conformer present at 25°C. A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, relative intensities, depolarization ratios and group frequencies. Several of the fundamentals for the gauche conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been obtained for both rotamers by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree–Fock (HF) and/or Møller–Plesset perturbation theory to the second order (MP2). Only with the 6-311G(2d,2p) and 6-311G(2df,2pd) basis sets with or without diffuse functions is the cis conformer predicted to be more stable than the gauche form. The potential energy terms for the conformational interchange have been obtained at the MP2(full)/6-311+G(2d,2p) level, and compared to those obtained from the experimental data. The results are discussed and compared to the corresponding quantities obtained for some similar molecules.
    Journal of Molecular Structure 01/2006; 786(1):9-24. · 1.40 Impact Factor
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    ABSTRACT: Infrared spectra of gaseous and solid 2-cyclopropylpropene (2-CPP, c-C3H5C (CH3)CH2) have been recorded from 3500 to 40 cm-1, and Raman spectra (3200-150 cm-1) of the liquid as well as mid-infrared spectra of 2-CPP in liquid krypton solution (from -105 to -150 degrees C) were also obtained. Ab initio calculations, with basis sets up to 6-311+G(2df, 2pd), were carried out for this molecule, using the restricted Hartree-Fock (RHF) approach, with full electron correlation by the perturbation method to second order (MP2(full)) and density functional theory (DFT) by the B3LYP method. The combination of the experimental and computational results (particularly with the higher basis sets) unequivocally identifies the more stable conformer of 2-CPP as the trans form, with the gauche rotamer higher in energy, but also stable. The cis structure of this compound is not observed experimentally, and is predicted by the computational approaches to be a transition state. By studying the temperature variation of two well-resolved sets of conformational doublets of 2-CPP dissolved in liquid krypton, an average enthalpy difference between conformers of 182+/-18 cm-1 (2.18+/-0.22 kJ mol-1) has been determined, with the trans conformation lower in energy in the fluid states, and the sole conformer present in the polycrystalline solid phase. This enthalpy difference corresponds to an ambient temperature conformational equilibrium in the fluid phases of 2-cyclopropylpropene containing approximately 55+/-2% of the more stable trans rotameric form. A complete vibrational assignment for the trans conformer of 2-CPP is given, and many of the bands of the gauche rotamer have also been assigned. Structural parameters, dipole moments, and rotational constants for this molecule have been calculated at the MP2(full)/6-311+G(d,p) level, and these results--as well as the results from the experimental studies--are compared to similar quantities in related compounds.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 06/2005; 61(7):1357-73. · 1.98 Impact Factor
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    ABSTRACT: The infrared (3200-40 cm(-1)) spectra of gaseous and solid 1,1-dicyclopropylethene, (c-C3H5)2C=CH2, along with the Raman (3200-40 cm(-1)) spectra of liquid and solid phases, have been recorded. The major trans-gauche (C=C bond trans to one ring with the other ring rotated about 60 degrees from the C=C bond, trivial C(1) symmetry) and gauche-gauche (the two three-membered rings rotated oppositely about 60 degrees from the C=C bond, C2 symmetry) rotamers have been confidently identified in the fluid phases, but no definitive spectroscopic evidence was found for the gauche-gauche' form (the two three-membered rings rotated to the same side about 60 degrees from the C=C bond, Cs symmetry), which is calculated to be present in no more than 6% at ambient temperature. Variable-temperature (-55 to -100 degrees C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. Utilizing six different combinations of pairs of bands from the C1 and C2 conformers, the average enthalpy difference between these two has been determined to be 146 +/- 30 cm(-1) (1.75 +/- 0.36 kJ x mol(-1)), with the C1 form more stable. Given statistical weights of 2:1:1 respectively for the C1, C2, and Cs forms, it is estimated that there are 75 +/- 2% C(1) and 19 +/- 1% C2 conformers present at ambient temperature. By utilizing predicted frequencies, infrared intensities, Raman activities, and band envelopes from scaled MP2(full)/6-31G(d) ab initio calculations, a complete vibrational assignment is made for the C1 form and a number of fundamentals of the C2 conformer have been identified. The structural parameters, dipole moments, and conformational stabilities have been obtained from ab initio calculations at the level of Hartree-Fock (RHF), the perturbation method to second order with full electron correlation (MP2(full)), and hybrid density functional theory (DFT) by the B3LYP method with a variety of basis sets. The predicted conformational stabilities from the MP2 calculations with relatively large basis sets are consistent with the experimental results. Structural parameters are estimated from the MP2(full)/6-311+G(d,p) predictions which are compared to the previously reported electron diffraction parameters. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.
    The Journal of Physical Chemistry A 04/2005; 109(8):1650-61. · 2.77 Impact Factor