Yu. A. Pentin

Joint treatment of ab initio and experimental data in molecular force field calculations with Tikhonov’s method of regularization

We describe a novel method for employing calculated ab initio potential data together with Tikhonov’s variational procedure to extract fundamental molecular force field parameters from experimental spectral data, the formal ‘‘inverse problem’’ of vibrational spectroscopy. In this approach, the ab initio quantities serve to ‘‘regularize’’ the initially ill‐posed problem (in the sense of Tikhonov), leading to variationally stable and unique force field parameters that optimally mimic overall patterns of the (approximate) ab initio quantities, but exactly reproduce the available experimental data within specified experimental precision. In this manner, ab initio and experimental data can be jointly combined to produce more stable and reliable force fields (improvable to any degree through higher level ab initio treatment, additional experimental data, etc.) than could be attained by theoretical or experimental methods alone. The proposed procedure allows use of any system of generalized coordinates, includin...

Vibrational spectra of trans,trans-, cis,cis- and cis.trans-1,4- dideuterobuta-1,3-dienes

Abstract The infrared and Raman spectra are reported for the trans,trans-, cis.cis - and cis,trans- 1,4- d 2 -buta-1.3-dienes. The proposed vibrational assignment is supported by normal frequency calculation.

Ab initio analysis of structure and vibrational spectrum of methyl nitrate

Abstract Ab initio calculations of the structure and harmonic force field of methyl nitrate have been computed at the HF/6-31G//HF/6-31G level. The assignment of the experimental frequencies of the CH 3 ONO 2 and CD 3 ONO 2 molecules were made using the unscaled quantum-mechanical force constants, which suggest a reassignment of the δ(NO 2 ) and ϱ r (NO 2 ) vibrations of CD 3 ONO 2 . Taking into account these new vibrational assignments, a refined harmonic force field for methyl nitrate is computed using a least-squares technique.

Vibrational spectra of RCH2HgX- and (RCH2)2Hg-typeorganomercury compounds

Abstract A study of organomercury compounds is reported. The IR and Raman spectra were measured and frequency assignments are made for 21 compounds. The characteristic frequencies of the -CH 2 HgX and the -CH 2 HgCH 2 -groups were determined and inferences are made on the molecular structure. Normal coordinate analysis was performed for diethylmercury, allylmercury chloride, allylmercury bromide, dibenzylmercury and benzylmercury chloride, bromide and iodide. The force constants in the environment of the Hg atom were refined by the least-squares method. It was found that the force constants of the C-Hg stretching vary in the range from 2.53 to 2.67 mdyn/A.

An ab initio calculation of the structure and scaled quantum mechanical harmonic force field of nitromethane

Abstract A complete optimization of the nitromethane geometry at the HF/6-31G and HF/ 6-31G* (5D) levels for staggered (one CH bond situated in a plane perpendicular to the plane of the heavy atoms) and eclipsed conformations is reported. The geometrical parameters obtained for the staggered conformation are in quite good agreement with the microwave structure (ref. 1). The effect of electron correlation on the energy difference between the two conformers is estimated using single point MP4SDQ/6-31G* calculations at the 6-31G geometry. The harmonic force field for both conformers are calculated at the HF/6-31G//HF/6-31G level. Taking into account the known overestimation of the vibrational frequencies by ∼20% in ab initio calculations, the experimental frequency assignment is confirmed. The six scale factors for the force field were evaluated using the experimental frequencies of nitromethane and nitromethane-d 3 (ref. 2). The vibrational frequencies for the five isotopomers of staggered nitromethane are determined with the scaled quantum mechanical force field (SQMFF). A detailed interpretation of conformational effects in the IR spectrum of crystalline CHD 2 NO 2 is proposed.

Ab initio and regularized force fields of haloethanes:CH3CH2Cl, CH3CHCl2, CH3CF2Cl, and CH3CFCl2

We present a systematic application of Tikhonov’s regularization method to joint treatment of ab initio and experimental vibrational data for a series of mono-, di-, and tri-substituted haloethanes, based on uniform MP2/6-31G* level of theory. This leads to MP2/6-31G*-“regularized” force field parameters that exactly reproduce experimental frequencies and isotopomer shifts (within prescribed error levels) and are “closest” (in Euclidean norm) to the ab initio force field. We correct a number of previous experimental spectral assignments and investigate the limits of transferability (e.g., of methyl-group force field parameters from one species to another) and other common simplifying approximations (e.g., MVFF modified valence force field treatment). Our results demonstrate how one can systematically combine experimental and ab initio information to create a data base of freon-type force field parameters having greater accuracy and reliability than could be obtained from theory or experiment alone.

Heat capacity from 12 to 300 K, phase transitions, and thermodynamic functions of 1,1,1-trifluoro-3-chloropropane and 1,1,1,3-tetrachloropropane

Abstract The heat capacity of 1,1,1-trifluoro-3-chloropropane and 1,1,1,3-tetrachloropropane from 12 to 300 K and also the temperature and enthalpy of fusion of these compounds were measured in a vacuum adiabatic calorimeter. The results obtained were used for calculating the thermodynamic functions from 12 to 300 K and, in conjunction with literature data, for calculating ΔG f o (l, 298.15 K). 1,1,1-Trifluoro-3-chloropropane was found to undergo two solid-phase transitions, one transition being monotropic ( T = (116.0 ± 0.1) K; ΔH ′ t = −(605 ± 9) cal th mol −1 ), and the other, enantiotropic ( T = (169.8 ± 0.1) K; ΔH ″ t = (1073 ± 4) cal th mol −1 ). Only one solid-phase transition was found for 1,1,1,3-tetrachloropropane ( T = (219.9 ± 0.1) K; ΔH ″ t = (527 ± 4) cal th mol −1 ). Raman, i.r., and n.q.r. spectra of various phases point to the association of the observed solid-phase transitions with the transformations of conformers.

Heat capacities, phase transitions, and thermodynamic functions of 1,1,2,2-tetrafluoro-1,2-dichloroethane and 1,1,2-trifluoro-1,2,2-trichloroethane

The heat capacities of 1,1,2,2-tetrafluoro-1,2-dichloroethane and 1,1,2-trifluoro-1,2,2-trichloroethane were measured from 8 to 300 K by adiabatic calorimetry and the thermodynamic functions were calculated from the results. Two phase transitions were found in the crystalline CF 2 ClCF 2 Cl. The conclusion was made, using i.r. spectra, that one of them: T ′ tr = (109.3 ± 0.2) K, Δ H ′ tr = (1212 ± 13) J mol −1 , was associated with the conformer transformations and the other: T ″ tr = (134.6 ± 0.1) K, Δ H ″ tr = (2628 ± 3) J mol −1 , with the formation of plastic crystal. Only one solid-phase transition was found in CF 2 ClCFCl 2 : T ″ tr = (82.5 ± 0.5) K, Δ H ″ tr = (830.9 ± 7.9) J mol −1 . This anomaly was probably caused by the onset of axial rotation of the molecules. The zero entropy of CF 2 ClCFCl 2 was found to be equal 10.1 J K −1 mol −1 . An assumption was made that a disorder of crystal II at T = 0 had a conformational nature.

Vibrational spectra and conformational analysis of five chlorosubstituted buta-1,3-dienes

Abstract A complete interpretation of the vibrational spectra of chloroprene, 2,3-dichlorobuta-1,3-diene, 1,1,2,3-tetrachlorobuta-1,3-diene, 1,1,2,3,4-pentachlorobuta-1,3-diene and hexachlorobuta-1,3-diene has been carried out and a normal coordinate analysis of these molecules has been performed. The conclusion about the non-planar conformation of the last three molecules was drawn from results of the CNDO/2 conformational calculations. The applicability of the CNDO/2 method to the molecules with the conjugated double bonds is discussed.

Vibrational spectra of trans, trans-, cis, cis- and trans, cis-1,4-dichlorobuta-1,3-dienes

Abstract The Raman spectra in liquid and solid phases and IR spectra in all states of aggregation are reported for the trans,trans-, cis,cis-, and trans,cis -1,4-dichlorobuta-1,3-dienes. A complete assignment of frequencies observed is given.