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Experimental (Raman and IR) and computational (DFT, MP2) studies of conformational diversity of 1-chloromethyl-1-fluorosilacyclohexane

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Abstract

Since 1-chloromethyl-1-fluorosilacyclohexane is a newly synthesized molecular compound its structural parameters and conformational stability is unknown. Raman and infrared vibrational spectroscopy methods were employed for analysis of this molecule. IR spectra were recorded for both gas phase and liquid sample, whereas the Raman experiments were performed in the liquid state. Additionally, low temperature matrix isolation infrared spectra were recorded after isolating the molecule in argon and nitrogen matrices. For the assignment of the experimental spectral bands, theoretical DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ calculations were performed. From the calculations it was found that 1-chloromethyl-1-fluorosilacyclohexane may exist in twelve different conformational forms out of which the chair axial trans conformer is the most stable form. However, there are three more chair type conformers – equatorial trans, equatorial gauche and axial gauche that are stable enough to be observed in the experimental spectra.

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... A comparison of the experimental IR and Raman results with the ones obtained from the calculations was also included in the study. Additionally, it was possible to study the influence of the size and electronegativity of the substituent atoms while com-paring current results with our previous studies, where an F atom and CH 2 Cl group are attached to the Si atom [52] . ...
... It is interesting to compare the results of this work with our recent studies of the 1-chloromethyl-1-fluorosilacyclohexane (1-ClM-1-FSiCH) molecule [52] , where the F (Fluorine) atom is bound to the Si contrary to the Cl (Chlorine) atom as it is in this work. F and Cl atoms differ in van der Waals radius (Cl-1.95 ...
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... It was observed that 1-chloromethyl-1fluorosilacyclohexane may exist in 12 different conformations, of which the chair axial trans conformer was found to be the most stable conformer. 10 It is worth noting that the aim of this study is not to investigate the conformational behavior of the chosen systems. However, the above citations have guided the understanding of the conformational behavior of the chosen systems and facilitated their geometry optimization to obtain the molecules to the correct energetic minima and conformation. ...
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Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
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In this paper, the N(+)-H···N, N(+)-H···O, and O-H···O(-) charge-assisted intramolecular hydrogen bonds (CAHBs) are investigated using different theoretical approaches. Monocharged cyclohexyldiamines (CHDA), aminocyclohexanols (ACHO), and cyclohexanediols (CHDO) are used as model compounds. Geometry optimizations at the MP2/aug-cc-pVDZ level are used to find the equilibrium structures for all possible H-bonded conformers. CAHBs are characterized geometrically and spectroscopically, and their energy is evaluated by means of homodesmic reactions. By comparison with the neutral forms, the presence of the charge is found to have a deep influence on the geometric and energetic H-bond parameters. In addition, these parameters are strongly dependent on the type of the groups involved as well as on their relative position in the cyclohexyl ring. For the systems under study, the H-bond energies vary from -23 to -113 kJ mol(-1), being classified from moderate to strong H-bonds. These H-bonds are also characterized by the application of the NBO and AIM theories. NBO analysis reveals that the energy corresponding to the charge transfer between the lone-pairs of the electron donor group and the antibonding orbitals of the acceptor group represents an important contribution in the H-bond stabilization. From the application of the AIM theory it is possible to see that these H-bonds possess some covalence which varies according to the type and relative position of the intervenient groups.
Article
We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar molecules cyclohexane (C(6)H(12)) and cyclohexane-d(12) (C(6)D(12)). We measured the rotational B(0) and centrifugal distortion constants D(J), D(JK) of the v = 0 states of C(6)H(12) and C(6)D(12) to high accuracy, for example, B(0)(C(6)H(12)) = 4306.08(5) MHz, as well as B(v) for the vibrationally excited states ν(32), ν(6), ν(16) and ν(24) of C(6)H(12) and additionally ν(15) for C(6)D(12). To successfully reproduce the experimental RCS transient, the overtone and combination levels 2ν(32), 3ν(32), ν(32) + ν(6), and ν(32) + ν(16) had to be included in the RCS model calculations. The experimental rotational constants are compared to those obtained at the second-order Møller-Plesset (MP2) level. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows determination of accurate semiexperimental equilibrium structure parameters, for example, r(e)(C-C) = 1.526 ± 0.001 Å, r(e)(C-H(axial)) = 1.098 ± 0.001 Å, and r(e)(C-H(equatorial)) = 1.093 ± 0.001 Å. The equilibrium C-C bond length of C(6)H(12) is only 0.004 Å longer than that of ethane. The effect of ring strain due to the unfavorable gauche interactions is mainly manifested as small deviations from the C-C-C, C-C-H(axial), and C-C-H(equatorial) angles from the tetrahedral value.
Article
Density functional methods were evaluated in their ability to predict relative conformational energies of a test set of monosubstituted cyclohexanes and six-membered heterocycles. It is shown that while popular density functionals like B3LYP are unreliable for predicting accurate conformational energies for the axial/equatorial equilibrium of monosubstituted cyclohexanes, 1-silacyclohexanes and tetrahydropyrans, density functionals that take into account dispersion interactions like M06-2X and B2PLYP-D result in energy differences close to CCSD(T)/CBS results. Using the M06-2X density functional, we have then investigated the conformational properties of a large number of monosubstituted silacyclohexanes, with the number of silicon atoms ranging from 1 to 6. Our calculations suggest remarkably different conformational properties when compared to cyclohexane. The carbon/silicon exchange in a cyclohexane ring often has systematic, yet counterintuitive effects on the conformational properties. Dispersion interactions are shown to be especially important for accurate relative energy calculations of polysilacyclohexanes.
Article
A perturbation theory is developed for treating a system of n electrons in which the Hartree-Fock solution appears as the zero-order approximation. It is shown by this development that the first order correction for the energy and the charge density of the system is zero. The expression for the second-order correction for the energy greatly simplifies because of the special property of the zero-order solution. It is pointed out that the development of the higher approximation involves only calculations based on a definite one-body problem.
Article
The features of blue- and red-shifted electron acceptor-donor (ACH/B) hydrogen bonds have been compared by using quantum chemical calculations. The geometry, the interaction energy and the vibrational frequencies of both blue- (ACH=F3CH, Cl3CH with B=FCD3) and red-shifted (ACH=F3CH, Cl3CH with B=NH3 and ACH=CH3CCH with B=FCD3, NH3) complexes were obtained by using ab initio MP2(Full)/6-31+G(d,p) calculations with the a priori basis-set superposition error (BSSE) correction method. One-dimensional potential energy and dipole moment functions of the dimensionless normal coordinate Q1, corresponding to the CH stretching mode of ACH, have been compared for both types of complexes. Contributions of separate components of the interaction energy to the frequency shift and the effect of electron charge transfer were examined for a set of intermolecular distances by using the symmetry-adapted perturbation theory (SAPT) approach and natural bond orbitals (NBO) population analysis.
Article
A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH(3)-, F-, OCH(3)-, and OH-substituted cyclohexane and tetrahydropyran rings. The high-accuracy relative electronic energies reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect. At the CCSD(T) complete basis set (CBS) limit, the energy of the equatorial conformation relative to the axial position (DeltaE (CBS)(CCSD(T))) is -1.75, -0.20, -0.21, and -0.56 kcal mol(-1) in methyl-, fluoro-, methoxy-, and hydroxycyclohexane, respectively, while DeltaE(CBS)(CCSD(T) is -2.83, +2.45, +1.27, and +0.86 kcal mol(-1) for 2-methyl-, 2-fluoro-, 2-methoxy-, and 2-hydroxytetrahydropyran, respectively. Note that the equatorial and axial conformers are nearly electronically isoenergetic in both fluoro- and methoxycyclohexane. For all eight cyclic species, a zero-point vibrational energy correction decreases Delta by a few tenths of a kilocalorie per mole. Relative energies obtained with popular methods and basis sets are unreliable, including Hartree-Fock theory, the B3LYP density functional, and the 6-31G and 6-311G families of split-valence basis sets. Even with the massive pentuple-zeta basis sets, the HF and B3LYP methods substantially overestimate the stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol(-1), respectively, for 2-methoxytetrahydropyran). Only because of a consistent cancellation of errors do these popular approaches sometimes provide reasonable estimates of the anomeric effect.
Article
The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol%/equatorial=42(12) mol%) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length Si--C 1.934(10) A. A low-temperature 19F NMR experiment results in an axial/equatorial ratio of 17(2) mol%:83(2) mol% at 113 K and a DeltaG (not equal) of 5.5(2) kcal mol-1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100-300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.
Relative Energies, Stereoelectronic Interactions, and Conformational Interconversion in Silacycloalkanes
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Spectroscopy, and Quantum-Chemical Calculations, Organometallics, 32, p. 6996−7005, 2013. (doi: dx.doi.org/10.1021/om4005725)
Vibrational spectra, conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane
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V. Aleksa, G. A. Guirgis, A. Horn, P. Klaeboe, R. J. Liberatore, C. J. Nielsen, Vibrational spectra, conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane, J. Mol. Struct., 61, p. 167-175, 2012. (doi: https://doi.org/10.1016/j.molstruc.2011.12.032)
Lopes jesus, J. S. Redinha, charge-assisted intramolecular hydrogen bonds in disubstituted cyclohexane derivatives
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