L. V. Khristenko

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.

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.

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The rg(C–F) bond is shortened to 1.307(13) Å in comparison with rg(C–F) = 1.356(4) Å in C6H5F.

Conformational composition of meta- and ortho-fluoro(trifluoromethoxy)benzene as studied by vibrational spectroscopy

The IR spectra of o- and m-F-C6H4OCF3 in the gas, liquid, and solid (glass and crystal) phases were analyzed along with the Raman spectra of these compounds in the liquid and solid (glass and crystal) phases. This investigation includes both experimental and theoretical studies of the spectra. Vibrational frequencies and normal modes for all possible stable conformers of m- and o-fluoro(trifluoromethoxy)benzene were calculated using B3LYP/cc-pVTZ harmonic quantum-chemical force fields. It was found that m-F-C6H4OCF3 exists as a mixture of the orthogonal and two planar conformers in the gas, liquid, and amorphous solid (glass) phases and as one orthogonal conformer in the crystal state, while o-F-C6H4OCF3 exhibits only one orthogonal conformer in every phase.

The molecular structure of 2-chloroacrolein studied by gas electron diffraction, vibrational spectroscopy and ab initio MO calculations

Abstract The molecular structure of 2-chloroacrolein was reinvestigated by means of gas-phase electron diffraction, ab initio MO calculations and vibrational analysis. From the MO calculations ( MP 2 6–31 G ∗ ), the molecule was found to exist as two conformers: an anti form which is more stable than its syn counterpart by 0.3 kcal mol −1 . The corresponding populations at room temperature are 70% and 30%, respectively. An analysis of the experimental vibrational spectra was carried out. The ab initio force field was scaled to fit the experimental spectra. The data obtained were used to calculate vibrational amplitudes and perpendicular corrections required for further electron diffraction analysis. In the electron diffraction analysis, the bond length and bond angle differences predicted by ab initio calculations of the conformers were assumed. The vapour was assumed to consist of two conformers: exact anti form and another close to syn form. The torsional angle of rotation about the CC bond in the latter was refined in two ways: the first refinement with perpendicular corrections and fixed amplitudes gave a dihedral angle in the range 160–180; the second, including both geometrical parameters and some amplitudes, resulted in a value of 160 for the dihedral angle. Conformational composition and geometrical parameters obtained from the electron diffraction analysis are in agreement with the results from ab initio calculations.

A reinvestigation of the molecular structure of dimethyl-N-nitramine by gas electron diffraction, ab initio calculations of the molecular geometry and the force field and vibrational spectra

Abstract The molecular structure of dimethyl- N -nitramine was reinvestigated with gas-phase electron diffraction (ED) and ab initio calculations. Ab initio calculations using different basis sets and HF, MP2 and DFT all predict a molecule with C s symmetry and a pyramidal amine N bond configuration. The vibrational spectra were interpreted from the scaling of the harmonic force field, and vibrational amplitudes required for the ED analysis were calculated from this scaled force field. The following values ( r g bond lengths in A and ∠ α angles in degrees with errors equal to three standard deviations) were found for the main parameters: r (N–O)=1.232(3), r (N–N)=1.387(3), r (N–C)=1.466(3), r (C–H) ave =1.114(9), ∠CCN=116.1(6), ∠CNC=122.4(27), ∠ONO=127.6(12), ∠NCH ave =109.9(18). The sum of the bond angles around the amine N atom is 354.6(28)°. The geometrical parameters obtained from the ED analysis are in agreement with the ab initio calculations except that a more pyramidal amine N bond configuration is predicted by ab initio.

Vibrational spectra and calculation of the noemal vibrations of 1,3-dimethyl-1,1,3,3-tetrachlorodisiloxane

We have recorded the IR spectra of DMTChDS in the gaseous, liquid, and crystalline states and the Raman spectra in the liquid and crystalline states (Fig. I). The frequencies and forms of the normal vibrations of this molecule have been calculated in order to provide a more reliable interpretation of the vibrational spectra. On account of the lack of data concerning the geometry of this molecule, data on hexachlorodisiloxane (HChDS) and hexamethy~disiloxaneo(HMDS) from [2, 3] were employed. The following parameters were used: RSi 0 1.60 A, RSi C 1.88 A, RSiCI 2.01 ~, RCH 1.09 ~, SiOSi 146 ~ The angles at the Si atoms were assumed to be tetrahedral. Retarded internal rotation around the Si--O bonds is possible in DMTChDS as a result of which this molecule can exist in the form of several conformers of C~, Cs, C2~ and C~ v symmetry. In carrying out the calculations it was assumed that one of the SiC1 or SiC bonds of the CH3CI2Si-groups lies on the SiOSi plane in a trans-position with respect to the SiO bond (Fig. 2) althongh in [2] it was concluded that there is some deviation from the planar structure of the trans-chain in HChDS. We carried out calculations for all four conformers using the one and the same force constant matrix which was set up using data on the force fields obtained in [4] for HChDS and H}fDS. No additional refinement of the force field was carried out. The results of the calculations are shown in Table i. The calculation enabled one to assign the experimental frequencies using the calculated forms of the vibrations. There is no doubt concerning the assignment of tNe frequencies of 2990 and 2918 cm -I to the ~(CH)as and ~(CH) s valence vibrations respectively and the frequencies of 1405 and 1270 cm -I to the ~(CH3)as and ~(CH3)s deformation vibrations. The absorption bands in the 760-850 cm -I region are assigned to p(CH3) and, moreover, the vibration with a frequency at around 800 cm -: is very strongly mixed with v(SiO) s. The antisymmetric v(SiO)as vibration appears in the spectrum as a strong broad band the position of which varies somewhat upon passing from the liquid to the crystal (1095 cm -~ in the liquid and 1145 cm -: in the crystal) which may be associated with an increase in the SiOSi angle in the crystal in comparison with this angle in the liquid [5-7]~ Two frequencies, 680 and 645 cm -~, may be assigned to the ~(SiO) s vibration. The first only appears in the IR spectrum as a very weak band which is most probably a combination frequency or an overtone and it is therefore more reasonable to assign the weak band at 645 cm -~ in the IR and Raman spectra to ~(SiO) s. It is partially polarized in the Raman spectrum. The weak, depolarized band in the Raman spectrum at 755 cm -~ is assigned to the v(SiC) valence vibration. The assignment of the valence vibrations of the SIC12 groups, the vibrational frequencies of which are shown in Table i, also does not present any significant difficulty. The assignment of the deformation vibrations of the CH3CI2Si-fragment is considerably more difficult. These vibrations are very strongly mixed with one another and any specific assignments can only be made provisionally (Table i).

VIBRATIONAL SPECTRA AND ROTATIONAL ISOMERISM OF BIS(N-2-CHLOROETHYL)NITRAMINE

The IR and Raman spectra of bis(N-2-chloroethyl)nitramine (BCENA) in the liquid and crystalline states and in CCl4 and CH3CN solutions are studied. The spectra are compared, and it is concluded that BCENA exists as a mixture of conformers of different polarities in the liquid state and as one less polar conformer in the crystalline state. To determine the conformations corresponding to the total electron energy minima and interpret the vibrational spectrum of BCENA, we performed an ab initio quantum chemical calculation of the BCENA molecule in the Hartree-Fock approximation using the 3–21G* and 6–31G* basis sets. Out of twelve possible conformations five are stable; the most stable conformer is C2(GG). The frequencies and forms of normal vibrations of stable conformers are calculated using scaled quantum chemical force fields. The calculated and experimental frequencies are compared, and the relations between the frequencies of skeletal stretching and bending vibrations are analyzed. It is concluded that the BCENA crystal is formed by the C2 (GG) conformer. The vibrational spectrum is interpreted, and the frequencies are assigned to vibrations of conformers differing in form.

The molecular structure of methyl(vinyl)-N-nitroamine reinvestigated by gas phase electron diffraction and ab initio MO calculations

Abstract The molecular structure of methyl(vinyl)- N -nitroamine has been reinvestigated by gas-phase electron diffraction and ab initio MO calculations. The methyl(vinyl)- N -nitroamine has been found from ab initio MO calculations to exist as a mixture of two conformers: an anti- and a syn-form, the former being more stable. The energy difference is predicted to be 3.3 kcal mol −1 . The results obtained by the calculation were used to evaluate vibrational amplitudes required for further electron diffraction analysis. In the electron diffraction analysis some bond length and bond angle differences found by ab initio calculations were assumed. The vapour was assumed to consist only of the anti-form. The torsional angle around the NC vinyl and NN bonds were refined. It has been found that the amine group is planar. The vinyl group is out of the molecular plane by 19° ( r a -structure).

Structure of the 3,3-dimethyl-3-silathietane molecule according to data from gas-phase electron diffraction analysis with consideration of vibrational effects

The 3,3-dimethyl-3-silathietane molecule has been investigated by gas-phase electron diffraction analysis with consideration of the vibrational effects. The following geometric parameters were obtained (the distances ra, the angles L~, the errors in the form of 3o in parentheses, for ~ 50): S-C 1.853(4), Si-C m 1.870(5) (m stands for methyl), Si-C r 1.916(3) (r stands for ring), C-H 1.086(3) ~, L(C-Si-C) m = 106.9(7), L(C-Si-C) r = 86.1(3), LSi-C-S = 90.5(5), and LC-S-C = 89.5(4) ~ The fourmembered ring is nonplanar; the angle 9 between the CSiC and CSC planes equals 2 0 . 3 ( 2 0 ) ~