High-resolution FTIR, microwave, and ab initio investigations of CH2 79BrF: ground, v(5) = 1, and v(6) = 1, 2 state constants.
ABSTRACT A spectroscopic study of CH279BrF in the infrared and microwave regions has been carried out. The rovibrational spectrum of the nu5 fundamental interacting with 2nu6 has been investigated by high-resolution FTIR spectroscopy. Owing to the weakness of the 2nu6 band, the v6 = 2 state constants have been derived from v6 = 1. For this reason, the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. Highly accurate ab initio computations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation. As far as the nu5 band is concerned, the analysis of the rovibrational structure led to the identification of more than 3000 transitions, allowing the determination of a set of spectroscopic parameters up to sextic distortion terms and pointing out first-order c-type Coriolis interaction with the v6 = 2 state. With regard to the pure rotational spectra measurements, the assignment of several DeltaJ = 0, +1 transitions allowed the determination of the rotational, all the quartic, and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole coupling tensor for both the ground and v6 = 1 states.
- SourceAvailable from: Malgorzata Biczysko[Show abstract] [Hide abstract]
ABSTRACT: This work aims to provide reliable benchmark data on the accuracy of harmonic and anharmonic vibrational frequencies computed with the B2PLYP double-hybrid density functional method. The exchange-correlation contributions required for the B2PLYP analytical second derivatives are presented here, which allow for the effective calculation of harmonic frequency as well as cubic and semidiagonal quartic force fields. The latter, in turn, are necessary to compute the anharmonic vibrational frequencies with the perturbative approach (VPT2). The quality of harmonic vibrational frequencies computed in conjunction with basis sets of double- to quadruple-ζ quality has been checked against reference data from the F38 benchmark set. Then, for an additional set of small closed- and open-shell systems, both harmonic frequencies and anharmonic contributions computed at the B2PLYP/N07D and the B2PLYP/aug-cc-pVTZ levels have been compared to their CCSD(T) counterparts. Moreover, for selected medium-size molecules (furan, pyrrole, thiophene, uracil, anisole, phenol, and pyridine), anharmonic frequencies have been compared to well established experimental results. Such benchmark studies have shown that the B2PLYP/N07D model provides good quality harmonic frequencies and describes correctly anharmonic contributions, the latter being of similar accuracy to their B3LYP/N07D counterparts, but obtained at significantly larger computational cost. Additionally, increased accuracy can be obtained by adopting hybrid models where the B2PLYP/N07D anharmonic contributions are combined with harmonic frequencies computed with more accurate quantum mechanical (QM) approaches or by B2PLYP with larger basis sets. This work confirmed also that most of the recently developed density functionals are significantly less suited for vibrational computations, while the B2PLYP method can be recommended for spectroscopic studies where a good accuracy of vibrational properties is required.Journal of Chemical Theory and Computation 06/2010; 6(7). · 5.31 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Impressive growth of computer facilities and effective implementation of very accurate quantum mechanical methods allow, nowadays, the determination of structures and vibrational characteristics for small- to medium-sized molecules to a very high accuracy. Since the situation is much less clear for open-shell species, we decided to build a suitable database of harmonic and anharmonic frequencies for small-sized free radicals containing atoms of the first two rows of the periodic table. The level of theory employed is the CCSD(T) model in conjunction with triple- and quadruple-ζ basis sets, whose accuracy has been checked with respect to the available experimental data and/or converged quantum mechanical computations. Next, in view of studies of larger open-shell systems, we have validated the B3LYP/N07D model with reference to the above database: our results confirm previous suggestions about the remarkable reliability and reduced computational cost of this computational method. A number of test computations show that basis set extension has negligible effects and other density functionals (including last generation ones) deliver significantly worse results. Increased accuracy can be obtained, instead, by using CCSD(T) harmonic frequencies and B3LYP/N07D anharmonic corrections.Journal of Chemical Theory and Computation 02/2010; · 5.31 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The enriched 81Br isotopic species of bromofluoromethane has been investigated in the infrared and microwave regions. The rovibrational spectrum of the ν5 fundamental has been studied by high resolution FTIR spectroscopy, while the rotational spectra of the ground and v6 = 1 states have been observed by means of microwave spectroscopy. More than 2700 transitions have been assigned in the ν5 band and the analysis of the rovibrational structure reveals a first-order c-type Coriolis resonance with the v6 = 2 state. The present study improves the ground state constants available in the literature and enables the determination of further centrifugal distortion parameters together with the full bromine quadrupole coupling tensor. A set of spectroscopic parameters up to the sextic distortion terms for the vibrational excited states has been accurately evaluated for the first time.Journal of Molecular Spectroscopy 11/2007; 246(1):126-132. · 1.67 Impact Factor