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The geometric structure of piracetam was studied by quantum chemical calculations (DFT and ab initio), gas electron diffraction (GED), and FTIR spectroscopy. Two stable mirror symmetric isomers of piracetam were found. The conformation of pyrrolidine ring is an envelope in which the C4 atom deviates from the ring plane, the angle between the planes (C3 C4 C5) and (C2 C3 C5) is 154.1°. The direction of the deviation is the same as that of the side acetamide group. The piracetam molecule is stabilized in the gas phase by an intramolecular hydrogen bond between the N9H2 group and the oxygen O6, bonded to C2. The principal structural parameters (re, A and ∠ e, degrees; uncertainties are 3σLS values) were found to be: r(С3 С4) = 1.533(1), r(C4 C5) = 1.540(1), r(N1 C5) = 1.456(1), r(C2 C3) = 1.520(1), r(N1 C7) = 1.452(1), r(C7 C8) = 1.537(1), r(N1 C2) = 1.365(2), r(C8 N9) = 1.360(2), r(C2 O6) = 1.229(1), r(C8 O10) = 1.221(1), ∠ C2 N1 C5 = 113.4(6), ∠ N1 C2 C3 = 106.9(6), ∠ N1 C7 C8 = 111.9(6), ∠ C7 C8 N9 = 112.5(6), ∠ N1 C2 O6 = 123.0(4), ∠ C3 N1 C7 = 120.4(4), ∠ C7 C8 O10 = 120.2(4), ∠ C5 N1 C2 O6 = 170(6), ∠ C3 C2 N1 C7 = 178(6), ∠ C2 N1 C7 C8 = 84.2, ∠ N1 C7 C8 O10 = 111.9. Download : Download full-size image

The molecular structure and conformational properties of 1,3-dimethoxybenzene (1,3-DMB) have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 methods with 6-31G(d,p) and cc-pVTZ basis sets). The differences between geometrical parameters were constrained at values calculated at B3LYP/cc-pVTZ and MP2/cc-pVTZ levels. Quantum chemical calculations predict three stable planar conformers for 1,3-DMB: syn–anti (Cs symmetry), anti–anti (C2v symmetry), and syn–syn (C2v symmetry) with abundances of about 65%, 20%, and 15%, respectively (syn and anti describe the orientation of the O–CH3 bonds relative to the C1–C2 and C2–C3 bonds, respectively). The GED analysis results in a mixture of 46(19)% syn–anti, 31(15)% anti–anti, and 23% syn–syn conformers, close to the contributions predicted by theoretical calculations. The experimental structural parameters agree well with results of B3LYP/cc-pVTZ and MP2/cc-pVTZ calculations.

The gas-phase electron diffraction experiment has shown that 1-monobromosilacyclobutane (MBSCB) exists in two conformational forms, the axial and equatorial with a significantly higher prevalence of the latter form (73(6)%). Various quantum mechanical procedures have been applied to investigate the thermodynamic equilibrium of the two conformers as well as the geometrical parameters of MBSCB. Among these methods was MP2/6-311++G(2df,2pd). This level of theory provided an axial to equatorial ratio of 29:71% and values for the geometrical parameters that are in good agreement with the experimental values except for the C–C bond length, which is by 0.02 A shorter than in the experiment. The main geometrical parameters obtained from the experiment are: ( r a A, ∠ a °): Si–C 1.872(3); C–C 1.583(6); Si–Br 2.225(3); ∠ CSiC 79.3; dihedral angle φ 28.8(52) ( axial ) and 39.9(2) ( equatorial ). Natural bond orbital (NBO) and atoms in molecule (AIM) analyses have been performed. Both, the donor–acceptor (Lewis and non-Lewis) orbital interactions and the topological properties of the charge density at the critical points ρ ( r ) have consistently confirmed the experimental results and facilitated their interpretation. For the purpose of comparison and systematic investigation, we optimized the geometries and analyzed the NBOs and the topological properties of silacyclobutane, 1-monofluorosilacyclobutane (MFSCB), and 1-monochlorosilacyclobutane (MCSCB). Simple relationship has been found between the puckering angle θ and the puckering amplitude q , which allows for the prediction of either θ or q for mono- and dihaloginated silacyclobutanes. Additionally, NBO deletion analysis comprising NOSTAR, NOVIC, and NOGEM deletion algorithms have been conducted. Interesting conclusions regarding structure and conformational stability of the studied monohalogenated silacyclobutanes could be drawn from this analysis.

For the first time, the five dimensional (5-D) analysis of potential energy surface (PES) from quantum-chemical calculations was carried out to predict reliably the various glyceraldehyde (GLA) conformers. 36 conformers with relative stabilities up to 38 kJ/mol were found in the B3LYP approximation. According to results of MP2/cc-pVQZ calculations, the molecule exists at the experimental temperature of 388 K as a mixture of five conformers in the ratio I:II:III:IV:V = 63:18:4:10:5. Contrary to the theoretical conclusion of Lovas et al., the conformer IV is predicted to be more stable than the conformer III. Our result can explain why the conformer IV could be detected in the microwave (MW) spectroscopic experiment by Lovas et al., whereas the conformer III could not. For the first time, thermal-average and equilibrium structural parameters of GLA (main conformer) have been determined from gas-phase electron diffraction (GED) data. Vibrational corrections to the experimental bond lengths were determined using quadratic and cubic force constants from high-level ab initio calculations (MP2/cc-pVTZ). It was shown that the experimental intensities are sensitive to the contribution of the second conformer (27(15)%). Rotational constants calculated from MP2/cc-pVQZ geometries were found to be in excellent agreement with the experimental rotational constants corrected for anharmonic effects.

The molecular structure and conformational properties of 2-methoxyphenol (2-MP) and 1,2-dimethoxybenzene (1,2-DMB) have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 methods with 6-31G(d,p) and cc-pVTZ basis sets). Of the three stable conformers predicted for 2-MP by quantum chemical calculations, the lowest energy form possesses a planar structure with an intramolecular hydrogen bond between phenolic hydrogen and methoxy oxygen ( anti–syn conformer of C s symmetry). The calculated concentration of this conformer is about 99% and this is confirmed by the GED data. Quantum chemical calculations predict three stable conformers for 1,2-DMB: anti–anti ( C 2v symmetry), anti–gauche ( C 1 symmetry), and gauche–gauche ( C 2 symmetry). The GED data were well reproduced for the mixture of these conformers with the relative abundance of 50 ± 12%, 36 ± 16%, and 14%, respectively. A similarly good agreement is also obtained for the single anti–gauche conformer. The experimental structural parameters agree well with results of B3LYP/cc-pVTZ calculations.

The geometric structure and conformation of (trifluoromethyl)thiobenzene, C 6 H 5 SCF 3 , has been determined by two independent gas electron diffraction (GED) studies at the Universities of Moscow and Ivanovo and by quantum chemical calculations. Both experimental studies result in perpendicular orientation of the S CF 3 bond relative to the benzene plane. This result is confirmed by several quantum chemical calculations (HF/6-31G(d), B3LYP/cc-pVTZ, MP2/6-31G(d), MP2/6-311+G(d,p) and MP2/cc-pVTZ) which predict single-minimum potential functions for internal rotation around the C(sp 2 ) S bond. A double-minimum potential function predicted by the B3LYP/6-31G(d) calculation is not compatible with the experiment. B3LYP and MP2 calculations with large basis sets (6-311+G(d,p) and cc-pVTZ) predict barriers to internal rotation around the C(sp 2 ) S bond between 2.4 and 2.6 kcal/mol. The geometric parameters of both experimental studies agree with each other within the experimental uncertainties, except for the C F bond distance. The effect of fluorination of the methyl group in thioanisole, C 6 H 5 SCH 3 , i.e., conformational change from planar orientation of the S CH 3 bond to perpendicular orientation of the S CF 3 bond, is rationalized on the basis of a natural bond orbital (NBO) analysis. The predominant effect is loss of conjugation between the p-shaped sulfur lone pair and the π system of the ring upon fluorination.

The geometric structure and conformational properties of thioanisole, C6H5SCH3, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations. The GED intensities are fitted very well with a single conformer with planar orientation of the S–CH3 bond (φ(C–S) = 0°). The experimental potential function for internal rotation around the C(sp2)–S bond derived with a dynamic model possesses a single minimum for planar conformation, but a second shallow minimum for perpendicular orientation cannot be excluded. Calculated potential functions depend strongly on the computational method. The MP2 approximation with small basis sets (6-31G(d)) predict a single minimum for perpendicular orientation, (φ(C–S) = 90°), with intermediate basis sets (6-311G(d,p)) an additional shallow minimum for planar orientation and with large basis sets (cc-pVTZ) the global minimum for planar orientation and an additional shallow minimum at φ(C–S) = 90°. The B3LYP/cc-pVTZ calculations result in a single minimum for planar orientation, in agreement with the GED experiment. The experimental and calculated barrier to internal rotation is about 1 kcal/mol. The following geometric parameters (ra and ∠h1 with 3σ uncertainties) were derived: r(C–C)av = 1.395(3) A, r(C(sp2)–S) = 1.775(4) A, r(S–CH3) = 1.813(4) A, ∠CSC = 104.5(5), ∠C2C1C6 = 119.3(3), ∠C1C2C3 = 120.2(4)°.

The molecular geometry of meta - and para -fluoronitrobenzene has been determined by gas electron diffraction and quantum chemical calculations using HF, MP2 and DFT, and 6-311G**, 6-311++G** and cc-pVTZ basis sets. The obtained geometrical parameters ( r g , in A, ∠ α in degrees) for meta -fluoronitrobenzene are: r (C–C) av =1.397(4), r (N–O) av =1.227(3), r (C–N)=1.484(3), r (C–F)=1.333(8), r (C–H) av =1.107(16), ∠C1C2N=118.3(6), ∠C5C6F=120.6(24), ∠ONO=125.3(37), ∠C1C2C3=122.9(24), ∠C3C4C5=119.2(18) and ϕ (C–N)=0 (fixed) and for para-fluoronitrobenzene: r (C–C) av =1.393(2), r (N–O) av =1.232(3), r (C–N)=1.479 (dependent), r (C–F)=1.338(12), r (C–H) av =1.124(27), ∠C6C4F=118.7 (fixed), ∠ONO=124.2(23), ∠C4C5C6=123.8(17), ∠C3C2N=119.1 (fixed) and ϕ (C–N)=0 (fixed). Estimated error limits are three standard deviations from least-squares refinement using a diagonal weight matrix. B3LYP/6-311G** has been used to calculate the molecular structure for all sixteen different molecules XC 6 H 5 (X=F, Cl, Br, I) and ortho -, meta - and para -XC 6 H 4 NO 2 (X=F, Cl, Br, I). These general trends are predicted by the calculations: the C–X bond in XC 6 H 5 is longer than the C–X bond in ortho -XC 6 H 4 NO 2 , the C–X bond in ortho -XC 6 H 4 NO 2 , is shorter than the C–X bond in meta -XC 6 H 4 NO 2 , the C–X bond in meta -XC 6 H 4 NO 2 is slightly longer than the C–X bond in para -XC 6 H 4 NO 2 , the variation in the C–X bond length is approximately the same for X=F and Cl, smaller for X=Br and smallest for X=I. These trends are essentially also found experimentally. The C–X bond lengths are all calculated too long compared to experimental values.

The molecular structure of the hydrogen bonded cyclic dimer of dimethylphosphinic acid (Me 2 P(O)OH) 2 was determined by gas-phase electron diffraction (GED) at 433 K. The presence of monomer cannot be determined at this temperature within the error limits for the GED method. Structural analysis was performed with consideration of non-linear kinematic effects at the first-order level of perturbation theory ( h 1). The vibrational characteristics of internuclear distances were calculated from a priori scaled quantum chemical (RHF/6-311G**) force field. The analysis aided by a constraint based on the RHF/6-311G** calculations yielded the following r h 1 -parameters of the C 2 -symmetry dimer configuration: PO 1.497(3); P–O 1.573(4); P–C 1.806(1) and 1.811(1) A; (C–H) av. 1.109(3) A; ∠O–PO 120(1)°. Unlike PO and P–O bonds, whose lengths in the gas phase and in the solid state differ insignificantly, the –O⋯O distance in the gas phase ( r h 1 2.81(4) A) is considerably longer than in the solid state ( r α 2.48(2) A). The latter is in accordance with the conclusion based on the IR spectra that transition from gas to a solid sample leads to strengthening of the H-bonds. Due to its small contribution to the diffraction pattern, the donor O–H bond length ( r h 1 0.99(1) A) was forcedly bound up with the parameters of C–H bonds. With this assumption, the other geometrical parameters characterizing the H-bond fragment have the following r h 1 values: O⋯H 1.84(4) A, ∠–OHO164(6)°, and ∠P–O–H 117(4)°. Conformational flexibility of the non-planar eight-atom ring of the dimer is experimentally verified by absence of any apparent peaks of the f ( r ) curve at the r -region of more than 4.2 A.

The geometry of silatrane HSi(OCH2CH2)3N has been determined by gas electron diffraction, ab initio calculations, and vibrational spectroscopy of crystal. Using the scaled force field from DFT calculations the amplitudes and perpendicular corrections were calculated. It was assumed that the silatrane molecule has C3 symmetry. The following values (rg bond lengths in A and ∠a bond angles in deg. with three standard deviations from the least-squared refinements using a diagonal weight matrix) are: SiN 2.406(27); NC 1.443(7); OC 1.399(11); SiO 1.648(3); CC 1.504(15); NSiO 78.8(21); SiOC 128.1(11); SiNC 105.4(14); CCO 117.0(26); CCN 108.2(30); CNC 113.2(17); OSiO 116.3(13). The 5-membered rings are flattened. The sum of its bond angles is equal to 537.5(42). It is shown that a very large difference is found for Si—N distance from ab initio and DFT calculating.

Gas electron diffraction data are applied to determine the geometrical parameters of the 1,1-dimethylsilacyclobutane molecule using a dynamic model where the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V ( ϕ )= V 0 [( ϕ / ϕ e ) 2 −1] 2 with V 0 =1.3±1.2 kcal/mol and ϕ e =29.7±4.5°, where ϕ is the puckering angle of the ring. A more reliable estimation of the barrier height ( V 0 =0.56 kcal/mol) was obtained by solving a one-dimensional Hamiltonian and fitting the transition frequency 0→2 to the experimental one. The geometric parameters at the minimum of V ( ϕ ) ( r a in A, ∠ α in degrees, and errors given as three times the standard deviations including a scale error) are: r (Si–C m )=1.872(2), r (Si–C r )=1.885(2), r (C–C)=1.563(4), r (C–H av )=1.115(3), ∠C m SiC m =109.9(47), ∠C r SiC r =79.2(11), ∠SiCH m =113.9(17), ∠SiCH eq =118.2(32), ∠SiCH ax =108.6(32), ∠CC 5 H eq =112.1(32), ∠CC 5 H ax =107.2(32), ∠HC 3 H=107.6, ∠ δ (C m SiC m )=3.9, ∠ δ (HC 3 H)=4.2, where the tilts δ and ∠HC 3 H are estimated from ab initio constraints, indexes m and r refer to methyl and ring. The structural parameters are compared with those obtained for related compounds.

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.

The molecular structure and conformational composition of tetrahydrofurfuryl alcohol in the gas-phase has been studied by a joint electron diffraction/ab initio method. The most abundant (84 ± 8%) conformer (A) in the gas-phase mixture was found to be stabilized by hydrogen bonding and had the O-H group placed over the furanose ring, which had a distorted 4 T3 conformation. The other conformer (B) with the relative abundance of 16 ± 8% had the O-H group located outside the ring and directed toward the ring oxygen also participating in the hydrogen bond formation. The barrier heights to pseudorotation in different conformers were estimated from ab initio calculations. Torsion strain was found to contribute mainly to the higher barrier near the E 1 form of the ring due to considerable ring flattening. The flattening might be a consequence of unfavorable axial position of the substituent at that point along the pseudorotation pathway. Differences between parameters in the same conformers as well as differences between parameters in the various conformers were assumed in the electron diffraction structure analysis from the MP2(FC)/6-31 I++G** ab initio calculations. The following values (rg bond lengths and r~ angles with total errors) were found for the main parameters in the most stable conformer: r(C--C)mea~ = 1.538 ± 0.004 A; r(C-O)m~ = 1.430 ± 0.003 ,~; r(f-H)mean = 1.109 -- 0.003 A; r(O-H) = 0.925 ± 0.012 A; LOI-C2- 6 = 105.5 ± 1.4°; LC3-C2-C 6 -- 112.3 +- 0.8°; LC2-C6-O7 = 112.9 ± 1.0°;/-C5-OrC2 = 113.1 ± 1.6°; LOrC2-C3 = 102.9 ± 0.6°; pseudorotation puckering amplitude q0 = 9.8 ± 0.3°; pseudorotation phase angle f= 85.5 ± 6.4 °. © 1998 Elsevier Science B.V. All rights reserved

The molecular geometry of 2-nitrotoluene has been determined by gas phase electron diffraction and quantum chemical computations at the MP2/6–31G∗ and Becke3-Lee-Yang-Parr (B3-LYP)/6–31G∗ levels of theory. Computed differences in CC bond lengths were utilized as constraints in the electron diffraction structure analysis. The scaled B3-LYP/6–31G∗ force field was used to generate the initial set of vibrational amplitudes. The electron diffraction study yielded the following bond lengths ( r g ) and bond angles: C 1 C 2 , 1.405(8) A; NO, 1.231(3) A; C 1 C 7 , 1.508(8) A; CN, 1.490(9) A; C 7 C 1 C 2 , 127.3(7)°; NC 2 C 3 , 113.8(6)°; C 1 C 2 C 3 , 124.2(9)°; C 6 C 1 C 2 , 114.8(6)°; C 5 C 6 C 1 , 123.1(10)°; O-N-O, 124.9(3)°; ϕ(CN), 38(1)°. The structural features of the molecule point to steric interactions prevailing between the methyl and nitro groups.

The molecular structures of gaseous o -fluorobenzaldehyde and m -fluorobenzaldehyde have been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations (at HF/6-331G∗∗ level). The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters ( r g )) from the joint analysis with estimated total errors (in parentheses) are for o -fluorobenzaldehyde: ( C  C ) mean = 1.3999(2) A , C  F = 1.334(5) A , C  C ( O = 1.515(6) A , C  O = 1.216(3) A , ∠CC F C = 122.0(2)°, ∠CC CHO C = 120.3(6)° and for m -fluorobenzaldehyde: ( C  C ) mean = 1.394(2) A , C  F = 1.346(4) A , C  C ( O = 1.494(4) A , C  O = 1.201(2) A , ∠CC F C = 122.3(1)°, and ∠CC CHO C = 120.6(3)°. The scaled molecular force fields have been determined.

The molecular structure of gaseous 4-fluorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF 6-311 G ∗∗ and MP 2 6-31 G ∗ levels of theory. The planar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters (rg) from the joint analysis with estimates of 2σ (in parentheses) were: ( CC ) mean = 1.397(1) A , CF = 1.331(7) A , CC (= O ) = 1.488(7) A , CO = 1.195(5) A

Gas electron diffraction data are applied to determine the geometrical parameters of the dichlorophenylphosphine molecule using a dynamic model where the rotation of the PCl 2 -group was treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree-Fock level of theory using a 6-31G∗∗ basis set. The internal rotation potential was found to have a unique minimum at ϕ = 0°, where ϕ is an angle between a bisector of the CIPCI angle and the plane of the benzene ring, and the potential has been described as the function V ( ϕ ) = 0.5 V 2 (1 − cos2 ϕ ) with V 2 = 1.24 ± 0.54 kcal mol −1 . The geometric parameters for the conformer at ϕ = 0° ( r a in A, ∠ α in degrees and errors given as three times the standard deviations including a scale error) are: r (PCl) = 2.060(3), r (PC) = 1.823(5), r (CC) = 1.396(2), r (CH) = 1.098(9), ∠CPCl = 100.5(5), ∠ClPCl = 100.2(5), ∠CC 1 C = 121.2(9), ∠CC 2 C = 119.5(8), ∠CC 6 C = 119.8(8), ∠ δ = 5.0(5), where δ is an angle between a PC bond and a bisector of the CC 1 C angle. The structural parameters are compared with those obtained for related compounds. The steric interactions are shown to have a more important effect on the geometry of the phenylphosphine derivatives than the p - π -conjugation.

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).

Structural parameters of o -chloroanisole have been determined using a dynamic model from electron diffraction together with vibrational spectroscopic data and ab initio calculations. A new approach to calculate the nonparametric torsional potential of the methoxy group based on Tikhonov's method of regularization was applied. The nonparametric torsional potential gives a two-minimum function with respect to the rotation energy around the CO bond. The most stable conformation corresponds to a planar form with the torsional angle ϕ = 0° and another to an orthogonal form with ϕ = 90°. The energy difference between these two conformers is found to be 0.9–1.0 kcal mol −1 and the barrier height at ϕ = 65° is 1.4–1.6 kcal mol −1 . These values are in good agreement with those calculated by the ab initio method. The main geometrical parameters ( r a in A, ∠ α in degrees and errors given as three times the standard deviation from least squares are as follows: r (CC) mean = 1.398(4); r (OC Ph ) = 1.358(36); r (OC Me ) = 1.426(21); r (OCl) = 1.733(4); r (CH) Ph = 1.086(6); r (CH) Me = 1.095(6); ∠CC O C Cl = 118.7(2.2); ∠C O CC = 119.9(2.5); ∠C O C Cl C = 121.5(1.1); ∠COC = 117.6(2.6); ∠C O CCl = 119.1(2.1); ∠CCO = 124.7(1.2). The structural results are compared with those obtained for similar compounds. Stereochemical peculiarities in ortho derivatives of anisole which result in the existence of an orthogonal conformer are discussed.

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 nonparametric numerical algorithm is suggested for calculation of the torsional potential from gas electron diffraction data using Tikhonov's method of regularization. A new approach to determine the admissible interval of the regularization parameter using Hamilton's statistical test has been considered. Applied to this reinvestigation of m -bromonitrobenzene it was confirmed that this algorithm only allows the determination of the form of the torsional potential function near its minimum. The barrier to internal rotation was estimated to be 4.6-5.4 kcal mol −1 as interpolated by Fourier series with proper parameterization. The most important structural parameters ( r a in Angstroms, ∠ α in degrees) are: r (CC) av = 1.399(3), r (CN) = 1.459(16), r (NO) = 1.244(3), r (CBr) = 1.884(6), r (CH) = 1.099(20), ∠CC N C = 123.9(1.4), ∠C N CC Br = 116.8(1.5), ∠C N CC = 116.6(1.9), ∠CNO = 118.8(0.8). The structure parameters are compared with those obtained previously for o -, m -, and p -bromonitrobenzenes.

The molecular structure of 3,3-dimethyl-3-silatetrahydrofuran has been studied by means of gas-phase electron diffraction (GED) and ab initio calculations at the HF/6-31G∗ level. Six different conformers of the title compound have been refined in the course of the GED analysis, allowing the vibrational amplitudes to vary over a wide range. Several bond length differences were taken from the results of the ab initio geometry refinement. Almost the same value of the R -factor, about 3%, has been obtained for all six models, whereas the ab initio calculation resulted in one single conformer (“O-envelope”). Based upon the combined information from the ab initio calculations and the GED work, the “O-envelope” conformer is chosen as the final structure. Geometrical parameters obtained (GED) are as follows (bond lengths in Angstroms, angles in degrees; three times standard deviation estimates in parentheses): r (SiC cyc ) = 1.891(3) and 1.904; r (SiC) Me ) = 1.877 and 1.879; r (CC) = 1.556(7); r (CO) = 1.431(3) and 1.445; r (C cyc H) = 1.11(2); r (C Me H) = 1.10(2); ф( O  C  C  Si ) = 33.0(14) ; ∠C cyc SiC cyc = 92.2(5); ∠C Me SiC Me = 107.2(8); ∠SiCC = 102.6(6); ∠CCO = 109.9(11); ∠HC cyc H = 110.5(47); ∠SiC Me H = 111.6(14).

An electron diffraction study of 5-methyl-2-furaldehyde (MFA) has been carried out to obtain information on its conformational properties and molecular geometry. Two conformers, with syn and anti orientations of the CO and CC bonds, were found to coexist with the composition 74(11) and 26%, respectively, at 333 K. The differences between the parameters in the syn and anti forms were assumed from ab initio calculations (MP2/6-31G∗) in the analysis. The following bond lengths ( r g , A) and bond angles (deg) were obtained for the syn form with estimated total errors parenthesized in units of the last digit of the parameter: CC(mean) = 1.366(14), CO = 1.223(4), OCC(Me) = 117.1(13), CO(mean) = 2.378(12), CH(Me) = 1.116(15), CCO = 110.5(5), CC(Me) = 1.507(14), COC = 105.7(11), CCO = 121.5(14), CC(ald) = 1.454(12), OCC(ald) = 115.8(18). The conformationals properties of MFA are similar to those of 2-furaldehyde while for acrolein the anti form was found to be the most stable.

The molecular structural parameters of divinyl ketone have been determined by gas electron diffraction and by ab initio MO calculations at the MP 2 6-31 G su∗ level. The MO calculations predicted the existence of two planar conformers of the molecule: a cis,cis (C,C) and a cis, trans (C,T), the former representing the global energy minimum. The energy difference between C,C and C,T was found to be 1.0 kcal mol −1 both at the SCF and MP2 calculational levels. In the analysis of the electron diffraction data, the theoretically predicted differences between corresponding bond lengths and valence angles in the C,C and C,T conformers were kept fixed. The bond lengths (in terms of r g , A) and the bond angles (in degrees) as obtained for C,C were as follows: r (CC) = 1.341(4), r (CC) = 1.483(4), r (CO) = 1.219(5), ∠CCC = 121.8(7), ∠CCH = 117.6(25), where numbers in parentheses are uncertainties expressed by 3σ. The relative abundance of the conformers (C,C:C,T), estimated from gas-phase electron diffraction, was 51:49 at 20°C with an uncertainty of about 21%.

The vibrational spectra of nitrobenzene and its para-d1, d5, 16O18O, 18O2 and 15N isotopic modifications are evaluated using the RHF/6-31G* ab initio harmonic force field. A rigorous interpretation of the experimental CNO2 moiety bands is carried out. Systematic deficiencies of the SCF method are effectively removed by applying scale factors optimized previously for a number of aliphatic nitro compounds. Fully optimized geometries are also reported for planar and orthogonal nitrobenzene conformations at the RHF and MP2 computational levels using the standard 6-31G* and 6-31G** basis sets. Theoretical geometries and barriers to internal rotation are compared with available experimental data. The calculations suggest that steric factors affect appreciably the structural parameters of the CNO2 fragment in the equilibrium planar conformation and consequently the potential function for internal rotation in nitrobenzene.

The molecular structure of trimethylgermylformate (TMGF) has been determined by electron diffraction in the gas phase. The principal parameters, bond lengths ( r a , A), bond and torsional angles (deg) with the least-squares standard deviations in parentheses, are as follows: GeC, 1.919(21); GeO, 1.820(28); CO, 1.213(76); CO, 1.326(42); CH(methyl), 1.109(80); C6H8, 1.12(fixed); GeOC, 117.6(23); OGeC, 104.9(7); GeCH, 109.8(80); OCO, 125.4(59); CGeC, 113.6(7); OCH, 127(fixed); GeOCO, 81.2(63); CGeOC, 35.5(38); HCGeO, 73.2(52). It is found that the GeOCO fragment of TMGF is not planar, and the HCO plane is close to the perpendicular position with respect to the GeOC plane. This result stands out from the series of previously obtained data for similar molecules and suggests that the conformation of TMGF is controlled mainly by steric effects. The conformation of TMGF from electron diffraction is in agreement with molecular mechanics calculations.

A harmonic force field for C6Cl3(NO2)3 has been estimated, and the calculated and experimental vibrational frequencies are compared. The vibrational amplitudes and shrinkage corrections have been calculated using the conventional technique based on normal coordinate analysis (1) and the new technique of taking into consideration non-linear terms in the transformation between the Cartesian and internal coordinates (2). The shrinkage corrections for bonded distances calculated using technique (2) are substantially (an order of magnitude) smaller than those calculated with the conventional procedure (1). The opposite effect is observed for non-bonded distances depending on internal rotations, which is more consistent with the physical sense of the influence of vibrational motions on the molecular structure.

The molecular structure 4,4′ -dinitrobiphenyl has been studied by gas-phase electron diffraction. The following geometrical parameters are obtained, assuming D 2 symmetry for the molecule as a whole (distances, r a , are given in A and angles, ∠ α , in degrees. The numbers in parentheses are standard deviation values): r(CC) av = 1.400(2); CC = 1.497(9); r(CN) = 1.477(6); r(NO) = 1.222(2); r(CH) av = 1.090(6); ∠CNO = 117.3(2); ∠C2C3C4 = 117.2(4); ∠C3C4C5 = 123.6(5); the angle between the planes of the two phenyl rings, 50.4 (1.7); the torsional angle of nitro groups, −25.0 (1.6). The results are compared with structural data for 4,4′-dinitrobiphenyl in the solid state and with data for molecules of related structure.

The molecular structure of bis ( N -chloromethyl) nitramine has been determined by gas electron diffraction. The bond lengths ( r g ), bond angles and angles of torsion (φ) with estimated total errors in parentheses are as follows: NO 1.216(3) A NN 1.422(5) A CN 1.443(4) A ClC 1.803(4) A CH 1.109(9) A ONO 128.0(14)° CNN 116.3(4)° CNC 124.8(9)° ClCN 111.6(5)° HCN 110.3(9)° φ CN 78.9(7)° φ NN 2.6(15)° The amine nitrogen has an essentially planar bond geometry. This molecule has practically C 2 symmetry. The two ClC bonds point to opposite sides of the planar C 2 NNO 2 skeleton.

The Raman (50–3200 cm−1) and infrared (50–3200 cm−1) spectra of bis(N-ethyl)nitramine, (CH3CH2)2NNO2, in the liquid and crystal states have been recorded. Optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing the RHF/6-31G∗∗ level. This compound was shown to have two stable conformations with a planar nitramine fragment and the CH3 groups orthogonal to it and located either on the same or on the different sides of it. The computed energy difference between two conformers is 0.57 kcal/mol. (CH3CH2)2NNO2 exists as a mixture of the two conformations in the liquid state, while only the most stable one, with the CH3 groups located on the different sides of the nitramine fragment, remains in crystal state. The vibrational frequencies have been calculated using ab initio scaled force fields, and the vibrational spectra have been interpreted in detail.