Igor F. Shishkov

The molecular geometries of some cyclic nitramines in the gas phase

The structural parameters of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), (CH2NNO2)3, 1,3-dinitro-1,3-diazacyclopentane (DDCP), CH2(CH2NNO2)2, andN-nitropyrrolidine (NP), (CH2)4NNO2, have been determined by electron diffraction.The six-membered ring of RDX has a chair form with axial positions of the nitro groups and close to planar bond geometry of the amine nitrogen atoms. The overallC3 symmetry of the molecule is in agreement with the experimental data.The conformation of the five-membered ring in DDCP is a half-chair ofC2 symmetry, while that in NP is an envelope ofCS symmetry. The nitro groups are in equatorial positions in both molecules. The conformations of pyrrolidine and imidazolidine cycles show interesting features.The pyramidal geometry of the amine nitrogen atom bonds flattens in going from pyrrolidine andN-chloropyrrolidine to NP and DDCP and then to RDX and to dimethylnitramine (DMNA), (CH3)2NNO2.

Molecular structure of ethyl nitrate from gas-phase electron diffraction and ab initio MO calculations

Abstract The molecular geometry of ethyl nitrate has been determined by gas-phase electron diffraction and ab initio MO calculations at the MP2/6-31G* level. There is reasonable general agreement between the experimental and computed structures and also with the rotational constants from a previous microwave study. Both anti and gauche forms are present at 310 K with the anti form prevailing. Computed differences between the parameters of the anti and gauche forms have been assumed from the ab initio results and the following bond lengths ( r g (A)) and bond angles (deg) obtained for the anti form: CO 1.444±0.004, NO 1.406±0.004, CC 1.520±0.004, NO (mean) 1.208±0.003, CON 113.0±0.6, ONO (anti) 112.2±1.0, ONO (syn) 118.2±0.7, CCO 106.0±1.0. The angles of torsion are φ (anti) 5.4°±3.0° and φ (gauche) 104°±8°; 0° corresponds to the anti form. The nitrogen bond configurations are similar in ethyl nitrate, methyl nitrate and nitric acid.

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.

Molecular geometry of 2-nitrotoluene from gas phase electron diffraction and quantum chemical study

Abstract 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.

Molecular structure and conformations of tetrahydrofurfuryl alcohol from a joint gas-phase electron diffraction and ab initio molecular orbital investigation

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 °.

Determination of the gas-phase molecular structures of tetraphenylsilane, tetraphenylgermane, and tetraphenyltin by electron diffraction

Abstract The structures of free Ph 4 M molecules (MSi, Ge, Sn) have been analysed by electron diffraction. Only a limited amount of reliable structural information could be determined since several models ( D 2d , S 4 , D 2 ) fit the experimental data equally well. The phenyl rings are slightly elongated. Assuming that b = c and γ = 120°, the r g parameters (with estimated total errors) have been obtained: 1 MC (CC) mean (CH) mean b  a α (A) (A) (A) (A) (deg.) Ph 4 Si 1.871(4) 1.403(3) 1.087(4) 0.005(5) 118.2(4) Ph 4 Ge 1.960(4) 1.401(3) 1.098(4) −0.012(6) 119.5(3) Ph 4 Sn 2.137(5) 1.401(3) 1.109(5) −0.037(8) 119.5(6) where a = r (C1-C2), b = r (C2-C3), c = r (C3-C4) α = ∠ C6-C1-C2, and γ = ∠ C2-C3-C4. The bond configuration of the central atom is tetrahedral, but the individual CMC bond angles as well as b - a and α are poorly determined because of their correlation with the conformations assumed in the analysis. Molecular parameters are consistent with those in the crystal phase.

Molecular structure of 3,3-dimethyl-3-silatetrahydrofuran from gas-phase electron diffraction analysis and ab initio calculations

Abstract 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).

Molecular geometry of 5-methyl-2-furaldehyde from gas electron diffraction

Abstract 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.

Molecular geometry of bis (N-chloromethyl)nitramine from gas electron diffraction

Abstract 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.

Inductive effects on bridging Ga-Cl distances: The molecular structure of the dichloro(methyl)gallium dimer, [Me2Cl2Ga2(μ-Cl)2], determined by gas electron diffraction

Abstract The gas-phase electron diffraction data for [Me 2 Cl 2 Ga 2 (μ-Cl) 2 ] are consistent with a trans model of C 2 h symmetry and bond distances ( r a ) GaC = 194.9(7) pm, GaCl t = 212.9(3) pm and GaCl b = 233.9(3) pm.