The molecular structure of trans-azobenzene (Ph-NdN-Ph) has been determined by gas electron diffraction. Diffraction patterns were taken at 407 K and data analysis was made using the structural constraints obtained from MP2/6-31+G* calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. Vibrational mean amplitudes were refined as groups. The torsion of each phenyl ring was treated as a large amplitude vibration. The potential function for torsion was assumed to be V(φ 1 ,φ 2 ), where φ i denotes the torsional angle around each N-C bond. Quantum mechanical calculations were performed by taking account of two torsional motions to derive a probability distribution function, P(φ 1 ,φ 2 ). Because P(φ 1 ,φ 2 ) ) N exp(-V(φ 1 ,φ 2 )/kT) was found to be a good approximation at 407 K where N is a constant, it was adopted in the data analysis. The determined potential constants (V 2 and V 4 /kcal mol -1 ) and principal structure parameters (r g /Å, ∠ R /deg) with the estimated limits of error (3σ) are as follows: V 2 ) 1.7(6); V 4 ) 0.6(13); r(NdN) ) 1.260(8); r(N-C) ) 1.427(8); ) 1.399(1); ) 1.102(7); ∠NNC ) 113.6(8); (∠NCC cis -∠NCC trans )/2 ) 5.0(9), where < > means an average value and C cis and C trans denote the carbon atoms cis and trans to the NdN bond, respectively. Thus, the stable form was found to be planar with C 2h symmetry. The observed structure was compared with those of trans-azoxybenzene (Ph-N(-O)dN-Ph) and transstilbene (Ph-CHdCH-Ph). The stability of the liquid crystals with these types of molecular cores was discussed on the basis of the gas-phase structures of the model compounds of cores. Nearly the same results were obtained in the data analysis using the constraints from RHF/6-31G** ab initio calculations.

The molecular structure of nicotine was determined by means of gas electron diffraction. The nozzle temperature was about 116 °C. The results of RHF, MP2, and DFT calculations were used as supporting information. The electron diffraction data were well reproduced by assuming the mixture of two conformers where both the methyl group and the pyridine ring are in the equatorial positions of the pyrrolidine ring. The determined structural parameters (r g and ∠α) are as follows:  〈r(C−N)pyrrol〉 = 1.462(4) Å; 〈r(C−C)pyrrol〉 = 1.541(4) Å; 〈r(C−N)pyrid〉 = 1.345(2) Å; 〈r(C−C)pyrid〉 = 1.397 Å (d.p.); r(Cpyrrol−Cpyrid) = 1.502 Å (d.p.); 〈r(C−H)pyrrol〉 = 1.116(4) Å; 〈r(C−H)pyrid〉 = 1.106 Å (d.p.); ∠C−N−Cpyrrol = 108.4(15)°; 〈∠N−C−Cpyrrol〉 = 102.4° (d.p.); 〈∠C−C−Cpyrrol〉 = 108.2(33)°; ∠C−N−Cpyrid = 116.8(2)°; 〈∠N−C−Cpyrid〉 = 124.1° (d.p.); 〈∠C−C−Cpyrid〉 = 118.3° (d.p.); ∠Npyrrol−Cpyrrol−Cpyrid = 116.8(10)°; ∠Cpyrrol−Cpyrrol−Cpyrid = 115.8(20)°; 〈∠H−C−H〉 = 111.9(41)°; α = 45.7(25)°; β = 32.9(38)°; φ = −87.7(74)°. Angle brackets denote averaged values, values in parentheses are 3 times the estimated standard errors referring to the last significant digit, and d.p. denotes dependent parameters. Angle α is the methyl out-of-plane angle, and β is the puckering angle of the pyrrolidine ring. Angle φ is the dihedral angle between the planes of pyrrolidine and pyridine rings. The distances between the two N atoms are determined to be 4.885 ± 0.006 and 4.275 ± 0.007 Å for the two conformers. By comparing these values with the distance between the N and carbonyl O atoms of acetylcholine, a strong nicotinic agonist, it is concluded that among the two conformers of nicotine, only the one with the longer N...N distance has the nicotinic activity.

As a model of the core of molecules forming liquid crystals, the molecular structure of phenyl benzoate (Ph-C(=O)-O-Ph) at 409 K was determined by gas electron diffraction, and the relationship between the gas-phase structures of model compounds and the nematic-to-liquid transition temperatures was studied. Structural constraints were obtained from RHF/6-31G ab initio calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coordinate analysis. Thermal vibrations were treated as small-amplitude motions, except for the phenyl torsion, which was treated as a large-amplitude motion. The potential function for torsion was assumed to be V(phi(1),phi(2)) = V(12)(1 - cos 2phi(1))/2 + V(14)(1 - cos 4phi(1))/2 + V(22)(1 - cos 2phi(2))/2, where phi(1) and phi(2) denote the torsional angles around the C-Ph and O-Ph bonds, respectively. The potential constants (V(ij)()/kcal mol(-)(1)) and the principal structure parameters (r(g)/A, angle(alpha)/deg) with the estimated limits of error (3sigma) are as follows: V(12) = -1.3 (assumed); V(14) = -0.5(9); V(22) = 3.5(15); r(C=O) = 1.208(4); r(C(=O)-O) = 1.362(6); r(C(=O)-O) - r(O-C) = -0.044 (assumed); r(C(=O)-C) = 1.478(10); = 1.396(1); angleOCO = 124.2(13); angleO=CC = 127.3(12); angleCOC = 121.4(22); ( angleOCC(cis) - angleOCC(trans))/2 = 3.0(15); ( angleC(=O)CC(cis) - angleC(=O)CC(trans))/2 = 4.8(17), where < > means an average value and C-C(cis) and C-C(trans) bonds are cis and trans to the C(=O)-O bond, respectively. The torsional angle around the O-Ph bond was determined to be 64(+26,-12) degrees. An apparent correlation was found between the contributions of the cores to the clearing point of liquid crystals and the gas-phase structures of model compounds of the cores of mesogens, i.e., phenyl benzoate, trans-azobenzene (t-AB), N-benzylideneaniline, N-benzylideneaniline N-oxide (NBANO), trans-azoxybenzene (t-AXB), and trans-stilbene. The structures of t-AB, NBANO, and t-AXB have been obtained by our research group.

As the first attempt to determine the gas-phase structure of molecules forming liquid crystals, the molecular structure of p-azoxyanisole (PAA, CH 3 O-C 6 H 4 -NOdN-C 6 H 4 -OCH 3 ), a mesogen, has been studied by gas electron diffraction. A high-temperature nozzle was used to vaporize the sample. The temperature of the nozzle was about 170°C. Structural constraints were taken from HF/4-21G(*) ab initio molecular orbital calculations on PAA. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coordinate analysis. The structural model assuming four conformers well reproduced the experimental data. Five bond distances, six bond angles, and two dihedral angles were determined. Mean amplitudes were adjusted in five groups. The dihedral angles between the phenylene rings and the azoxy plane have been determined to be 11(26)°and 11(11)°, and these values are in agreement with those in the solid phase determined by X-ray diffraction within experimental errors. The conformation of the core of this mesogen is mainly ascribed to the interaction between the π-electrons of the azoxy group and the aromatic rings.

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