Effects of molecular structure on the stability of a thermotropic liquid crystal. Gas electron diffraction study of the molecular structure of phenylbenzoate
T. Tsuji et al., Effects of molecular structure on the stability of a thermotropic liquid crystal. Gas electron diffraction study of the molecular structure of phenylbenzoate, J AM CHEM S, 123(26), 2001, pp. 6381-6387
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 structure
s of model compounds and the nematic-to-liquid transition temperatures was
studied. Structural constraints were obtained from RHF/6-31G** ab initio ca
lculations. Vibrational mean amplitudes and shrinkage corrections were calc
ulated from the harmonic force constants given by normal coordinate analysi
s. Thermal vibrations were treated as small-amplitude motions, except for t
he phenyl torsion, which was treated as a large-amplitude motion. The poten
tial function for torsion was assumed to be V(phi (1),phi (2)) = V-12(1 - c
os 2 phi (1))/2 + V-14(1 - cos 4 phi (1))/2 + V-22(1 - cos 2 phi (2))/2, wh
ere phi (1) and phi (2) denote the torsional angles around the C-Ph and O-P
h bonds, respectively. The potential constants (V-ij/kcal mol(-1)) and the
principal structure parameters (r(g)/Angstrom, angle (alpha)/deg with the e
stimated limits of error (3 sigma) 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); (r (C-C))= 1.
396(1); angle OCO = 124.2(13); angleO=CC = 127.3(12); angle COC = 121.4(22)
; (angle OCCcis - angle OCCtrans)/2 = 3.0(15); (angleC(=O)CCcis - angleC(=O
)CCtrans)/2 = 4.8(17), where ()means an average value and C-C-cis and C-C-t
rans bonds are cis and trans to the C(=O)-O bond, respectively. The torsion
al angle around the O-Ph bond was determined to be 64(+26,-12)degrees. An a
pparent correlation was found between the contributions of the cores to the
clearing point of liquid crystals and the gas-phase structures of model co
mpounds of the cores of mesogens, i.e., phenyl benzoate, trans-azobenzene (
t-AB), N-benzylideneaniline, N-benzylideneaniline N-oxide (NBANO), trans-az
oxybenzene (t-AXB), and trans-stilbene. The structures of t-AB, NBANO, and
t-AXB have been obtained by our research group.