久世 信彦

The gas-phase structure of dichloroacetaldehyde oxime (Cl2CH-CH=NOH, DCAO), was determined by gas-phase electron diffraction (GED) combined with microwave (MW) spectroscopic data. The nozzle temperature in the GED experiment was about 53 degrees C. Structural constraints in the GED data analysis were obtained by the ab initio MO calculation of DCAO at the MP2/6-31G(d, p) level of theory. Vibrational amplitudes, shrinkage corrections for the data analysis of GED and vibrational corrections of the experimental rotational constants were calculated from the harmonic force constants given by normal coordinate analysis. The (E)-isomer with the dihedral angle of phi(ClCCN) = 119.7(2)degrees was the dominant conformer. The MW spectroscopic investigation and the optimized structure in the ab initio calculations were consistent with the present result. The population of the dominant conformer was 80(1)%. The results of the data analysis indicated that there were two other conformers involved, whose conformations and populations were: (E)-isomer with one chlorine atom on the plane of the molecular skeleton (10(1)%) and (Z)-isomer with phi of about 105 degrees (10(1)%). The principal bond distances and angles (r(g)/Angstrom and angle(alpha)/deg) of the dominant conformer, (E)-isomer, were: r(C-C) = 1.497(8); r(C=N) = 1.281(4); r(C-Cl) = 1.784(2); r(N-O) = 1.415(4); angle CCN = 117.0(8); angle CCCl = 109.4(3); angle CNO = 111.1(5); and angle NOH = 97.2(50). The values in parentheses were three times the standard deviations. (C) 1999 Elsevier Science B.V. All rights reserved.

The microwave spectrum of (E)-benzaldehyde oxime, C6H5-CH=NOH and C6H5-CH=NOD, has been observed in the frequency range from 26.5 to 40.0 GHz. The spectrum of the ground vibrational state was assigned and fitted to the Watson's A-reduced Hamiltonian to obtain these rotational and centrifugal distortion constants: A = 5183.13(29) MHz, B = 895.367(3) MHz, C = 763.819(3) MHz, Delta(J) = 0.019(3) kHz, and Delta(JK) = 0.204(7) kHz for the normal species, and A = 5158.4(23) MHz, B = 869.44(2) MHz, C = 744.34(2) MHz, Delta(J) = 0.023(2) kHz, and Delta(JK) = 0.193(7) kHz for the deuterated species. The values of the Delta I (=I-c - I-a - I-b) obtained for the normal and deuterated species were -0.295(6) and -0.28(5) a.m.u. Angstrom(2), respectively. The molecular conformation of this molecule was a planar one in which the values of the dihedral angles, CCNO and CNOH, were almost 180 degrees and 180 degrees, respectively. (C) 1999 Academic Press.

We have chosen a few linear molecules [NCCCX, X = H, F, Cl, Br; FCCH, FCCF, ClCCCl], containing electronegative atoms to study the structure and properties of these molecules by using a higher level of DFT and ab initio methods. The molecular geometries were optimized employing the atomic basis sets 6-311++G(2d,2p), 6-311++G(3df,3pd) in BLYP, B3LYP, BP86 level of theory of DFT and 6-13G* basis set at MP2 level of theory in the ab initio method. The optimized structural parameters and other properties of the molecules obtained by the post-Hartree-Fock and DFT methods are discussed with experimental values. Density functional theory parameters chemical hardness and chemical potential were calculated employing the atomic basis set 6-13G* at the MP2 level of theory in the ab initio method. These parameters were discussed in relation to the electronegativity of the atoms. (C) 1999 Elsevier Science B.V. All rights reserved.

The molecular structure of a typical mesogen, 4-methoxybenzylidene-4'-n-butylaniline (MBBA, CH(3)O-C(6)H(4)-CH=N-C(6)H(4)-(CH(2))(3)-CH(3)). has been studied by gas-phase electron diffraction (GED). The nozzle temperature was about 150 degrees C. Structural constraints in the GED data analysis were obtained by the ab initio MO calculation at the HF/4-21G(*) level of theory. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. The phenylene ring attached to the C(=N) atom and the azomethine group (-CH=N-) are essentially on the same plane, i.e., the dihedral angle is 0(12)degrees. The phenylene ring bonded to the nitrogen atom is found to be out of the plane of the azomethine group and the determined value of the dihedral angle, 48(9)degrees, in the gas phase is larger than that in the crystalline state. This is mainly due to the steric interaction between the hydrogen atoms of the azomethine group and the phenylene ring. In the gas phase the four rotational conformers with respect to the configurations of the n-butyl group were assumed to exist. Their conformational abundance was fixed, as calculated from the ab initio relative energies. The principal bond distances and angles (r(g)/Angstrom and angle(alpha)/deg) determined by GED are r(N=C) = 1.290(12), r(C-N) = 1.413(12), r(C(az)-C(ring)) = 1.467(3), [r(C(ring)-C(ring))] = 1.400(6), angle C-N=C = 119.0(18), angle N=C-C = 121.6(13), angle NC(ring)C(14) = 128.5(25), angle C(az)C(ring)C(4) = 121.2 (dependent), [CCC(ring)] = 120.0(3), [angle CCC(butyl)] = 116.2(11), angle C(5)C(ring)O = 129.3 (16), where C(az), C(ring), and C(butyl) denote the carbon atoms of the azomethine, phenylene and butyl groups, respectively. C(14) and C(4) are the C atoms of the rings synclinal to the C(=N) atom and cis to the H(az) atom, and C(5) is the C atom of the ring cis to the C atom of the methoxy group. The values' in parentheses are three times the standard deviations. The notation [ ] represents the average value. The transition temperature from the nematic to liquid phases was discussed on the basis of the determined molecular structure.

The microwave spectra of dichloroacetaldehyde oxime ((Cl2CHCH)-Cl-35=NOH) and its two isotopic species ((ClClCHCH)-Cl-37-Cl-35=NOH and (Cl2CHCH)-Cl-35=NOD) were observed in the frequency range from 26 500 to 40 000 MHz. Their rotational constants and centrifugal distortion constants were determined: for the normal species in the ground vibrational state, A = 3149.619(242) MHz, B = 1368.304(13) MHz, C = 996.960(5) MHz, and Delta(JK) = 3.479(89) kHz. By referring to the values of the rotational constants and the planar moments (P-bb) and the r(s) coordinates of the chlorine and hydroxyl hydrogen atoms, we deduced the conformation of the molecule to be (E)-isomer with the ac C-s symmetry plane. The bond length C-Cl, bond angle CICC, and dihedral angle ClCCN, were determined to be 1.763(19) Angstrom, 110.4(17)degrees and 117.9(26)degrees, respectively. These values were in good agreement with those calculated by the ab initio MO calculations (MP2/6-31G** level). The C-C torsional frequency of (E)-dichloroacetaldehyde oxime was estimated to be 77(35) cm(-1) from the relative intensity measurements. (C) 1999 Elsevier Science B.V. All rights reserved.

The microwave spectrum of n-butyraldehyde oxime was observed in the frequency region 26.5-40 GHz. Four rotational conformers were found to exist in the gas phase; among these, two conformers belonged to the (E)-geometrical isomer and the other two to the (Z)-geometrical isomer. The microwave spectrum attributed to one of these two rotational conformers of (E)-butyraldehyde oxime was analyzed, and its rotational constants for the ground vibrational state were determined: A = 15883(379), B = 1269.97(1), C = 1251.60(1) MHz. The conformational structure of the molecule is discussed, referring to the rotational constants obtained and the ab initio molecular orbital calculation. (C) 1998 Academic Press.

The microwave spectra of (E)-(ClCH2CH)-Cl-35=NOH, (E)-(ClCH2CH)-Cl-37=NOH, (E)-(ClCH2CH)-Cl-35=NOD, and (E)-(ClCH2CH)-Cl-37=NOD have been observed in the frequency range from 26.4 to 40.5 GHz. The rotational and centrifugal distortion constants of the four isotopic species in the ground and excited vibrational states were determined, The values of the Delta I(=I-f -I-a -I-b) obtained for the normal (Cl-35 and Cl-17) and deuterated (Cl-35 and Cl-37) species in the ground vibrational state were found to be -16.350(4), -16.36(4), -17.06(4), and -17.07(6) u Angstrom(2), respectively. The molecular structure of this molecule was determined to be the anticlinal form (phi(1):angle ClCCN = 121.4 degrees, phi(2):angle CNOH = 180.0 degrees), as shown in Fig. l(a). It was found that ab initio calculation of MP2/6-31G** level can be used to predict the most stable rotational conformer of (E)-ClCH2CH=NOH, The seven structural parameters of the heavy atom skeleton of this molecule were fitted to the eight rotational constants (B and C) of the four isotopic species. The obtained structural parameters (r(0)) are almost the same as those (r(alpha)) Obtained previously by a gas-phase electron diffraction study (K. Iijima, T. Miwa, T. Sakaizumi, O. Ohashi, J, Mol. Struct. 352/353 (1995) 161), within the errors. The r(C=N) and angle CCN obtained for (E)-ClCH2CH=NOH are slightly shorter and drastically narrower than those of (Z)-ClCH2CH=NOH. The trend is consistent with those of CH3CH=NOH and CH3CH2CH=NOH. (C) 1997 Elsevier Science B.V.

The pyrolysates of bicyclo[2.2.1.]hept-2-ene and its derivatives have been investigated by pyrolysis-mass spectrometry and microwave spectroscopy (Py-MS/MW). The pyrolysates of ethene, s-trans-acrolein, methyl vinyl ketone, methylallene, and isocyanic acid were generated by elimination of cyclopentadiene(Cp) from bicyclo[2.2.1.]hept-2-ene, 2-carboxaldehyde-bicyclo[2.2.1]hept-5-ene, 2-acetyl-bicyclo[2.2.1.]hept-5-ene, 2-ethylidene-bicyclo[2.2.1.]-hept-5-ene, and 2-azabicyclo[2.2.1.]hept-5-ene-3-one, respectively. (C) 1997 Elsevier Science B.V.

Molecular structure and conformational stability of E and Z geometrical isomers of propionaldehyde oxime (C(1)H3C(2)H2C(3)HNOH) have been studied by using the ab initio and DFT methods. The molecular geometries were optimized by employing the atomic basis sets 6-31G* at the HF-SCF and MP2 levels of theory in the ab initio method. The basis sets 6-31G and 6-31G* are used in the BLYP method of DFT to optimize the molecule. The optimized structural parameters of the above methods are discussed in the light of the electron diffraction results of the molecule. The variations in C=N bond length, C1C2C3 and C2C3M angles of the sp and ac conformers of E isomer and of the ap conformer of Z isomer have been discussed in terms of nonbonding interactions of CH3 and NOH groups. The rotational potential energy surfaces of E and Z isomers were obtained for the C2-C3 rotational angle of propionaldehyde oxime at HF/6-31G*, MP2/6-31G*, and BLYP/6-31G* levels of theory. The global minimum occurs at phi(CCCN) = 120 degrees and phi = 0 degrees for the ac and sp conformations of the E isomer and phi = 180 degrees for the Z isomer. The ac form is found to be more stable than the sp form by 0.16 kcal/mol in HF/6-31G* level of theory; this difference agrees very well with the experimental value of 0.15 kcal/mol. The rotational potential curve of Z form shows that it has large-amplitude motion. The chemical hardness values obtained for the different conformers of the two isomers are in disagreement with the statement that the higher stable conformation has higher chemical hardness, but the trend obeys the general trend of oxime molecules. The Fourier decompositions of the rotational potential of the propionaldehyde oxime are analyzed.

The molecular structure of 3-methylthiophene [GRAPHICS] has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3-21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r(g)(S-C-2) = 1.719(2) Angstrom, r(g)(C-2 = C-3) = 1.370(3) Angstrom, r(g)(C-3-C-6) = 1.497(6) Angstrom, r(g)(C-2-H) = 1.101(5) Angstrom, angle C3C = 91.6(2)degrees, angle SC2C3 = 113.3(5)degrees, angle SC5C4 = 111.3(3)degrees, angle C2C3C6 = 123 2(11)degrees and angle C3C6H = 112(2)degrees. The values of r(S-C-2) - r(S-CS) and r(C-2=C-3) - r(C-4=C-5) were fixed at the 3-21G* value of 0.002 Angstrom. Parenthesized values are the estimated limits of error (3 sigma) referring to the last significant digit.

The molecular structure and conformation of diethyl ketone (3-pentanone), C(3)H3C(2)H2C(1)OC(4)H2C(5)H-3, at 27 degrees C were studied by gas electron diffraction (GED). Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Structural constraints were obtained from RHF/6-31G* ab initio calculations. Diffraction data were reproduced best by a mixture of two conformers, trans-trans (TT, dihedral angles phi(1)(C4C1C2C3) = phi(2)(C2C1C4C5) = 180 degrees) and trans-gauche (TG). The structural parameters of the TT form determined by GED, with error estimates (3 sigma) in parentheses, are: r(g)(C=O)=1.217(2)Angstrom, r(g)(C-1-C)=1.519(1)Angstrom, r(g)(C-2-C-3)=1.524(1)Angstrom, [r(g)(C-H)] = 1.103(3)Angstrom, angle(alpha)CC(=O)C=116.0(6)degrees, angle(alpha)C(=O)CC=114.2(6)degrees, angle(alpha)C(=O)CH= 108.2(7)degrees, and (angle(alpha)C(2)C(3)H)= 111.4(7)degrees, where [] denotes average value. The difference between r (C-1-C) and r(C-2-C-3) was assumed. Similarly angle C(=O)CH and angle C2C3H were refined in groups. The structural differences between the TT and TG conformers were taken from the 6-31G* calculations except for the dihedral angles. The dihedral angles of the TG form were determined to be phi(1) = 61(7)degrees and phi(2) = 151(14)degrees. The relative abundances of the TT and TG conformers are 50(10) and 50%, respectively.

The molecular structure of methyl isonicotinate was studied by gas phase electron diffraction combined with ab initio calculations. The molecular skeleton was assumed to be planar. The determined values of principal structure parameters (r(g) and angle(alpha)) are as follows: r(N-C) = 1.343(5) Angstrom, r(CC)(ring) = 1.401(3) Angstrom, r(C-gamma-C) = 1.499(9) Angstrom, r(C=O) = 1.205(5) Angstrom, r (C(=O)-O) = 1.331(8) Angstrom, r(O-C-Me) = 1.430(8) Angstrom, (r(C-H)) = 1.103(10) Angstrom, angle CNC = 117.6(9)degrees, LC(beta)C(gamma)C(beta) = 118.7(9)degrees, angle Cbeta,transCgamma-C(=O) = 118.6(12), angle CgammaC=O = 121.4(12)degrees, angle CgammaC-O = 114.2(10)degrees, angle COC = 115.4(15)degrees, where angled brackets denote average values and C-beta trans denotes the carbon atom which is trans to the carbonyl oxygen atom. Values in parentheses are the estimated limits of error (3 sigma) referring to the last significant digit. The structure of the ring in methyl isonicotinate agrees with that of pyridine within experimental error. In contrast, the structure parameters of the COOCH3 group are significantly different from those of methyl acrylate and methyl acetate. These differences have been discussed in terms of hyperconjugation and steric effects.

The molecular structure of 2-methylthiophene {A figure is presented} has been determined by gas electron diffraction combined with microwave spectroscopic data. Ab initio calculations at treated as a large-amplitude motion. A C6{single bond}H bond was found to be cis with respect to the C{double bond, long}C bond in the equilibrium state. The structural parameter values with estimated error limits (3σ) in parentheses are as follows: rg(S{single bond}C2) = 1.729(1) Å, rg(C2{double bond, long}C3) = 1.374(3) Å, rg(C2{single bond}C6) = 1.505(5) Å, rg(C5{single bond}H = 1.096(5) Å, ∠CSC = 92.7(2)°, ∠SC2C3 = 110.5(4)°, ∠SC5C4 = 111.0(8)°, ∠SC2C6 = 121.4(6)°, t = 4(5)° where t is the tilt angle which is defined to be the angle between the C3 axis of the methyl group and the C2{single bond}C6 bond. The values of r(S{single bond}C2) - r(S{single bond}C5), r(C2{double bond, long}C3 - r(C4{double bond, long}C5), r(C3{single bond}H) - r(C5{single bond}H), r(C4{single bond}H) - r(C5{single bond}H), r(C6{single bond}H) - r(C5{single bond}H), ∠C{double bond, long}CH, ∠SCH and ∠HCH were taken from the HF/3-21G* calculations. © 1995.

The molecular structure and conformation of diethyl ether, C(3)H3C(2)H,O(1)C(4)H2C(5)H-3, at 27 degrees C was studied by a joint analysis of gas electron diffraction and microwave spectroscopic data. Vibrational mean amplitudes and shrinkage corrections were calculated on the basis of the vibrational spectra measured in the vapor and liquid phases. Ab initio geometry optimization at the HF/4-21G level was performed on four possible conformers and the results were used as structural constraints. Diffraction and vibrational spectroscopic data are consistent with the presence of two conformers, trans-trans (TT, dihedral angles phi(1)(C4O1C2C3) = phi(2)(C2O1C4C5) = 180 degrees) and trans-gauche (TG), in the vapor phase. The compositions of the TT and TG conformers were determined to be 69(8) and 31%, respectively, by thejoint analysis. The structural parameters of the TT form determined are r(g)(O-C) = 1.419(1) Angstrom, r(g)(C-C) = 1.514(2) Angstrom, (r(g)(C-H)) = 1.114(2) Angstrom, angle(alpha)COC = 113.5(4)degrees, angle(alpha)OCC = 108.6(1)degrees, angle(alpha)OCH = 110.4(9)degrees, and (angle(alpha)CCH) = 110.5(7)degrees, where values in parentheses are error estimates (3 sigma) and those in angle brackets denote average values. The structural differences between the TT and TG conformers were taken from the 4-21G calculations except for the dihedral angles of the TG form, which were determined to be phi(1) = 76(10)degrees and phi(2) = 161(15)degrees.

The structural and conformational properties of N-chloro-N-ethylethanamine, C(3)H3C(2)H2N(1)ClC(4)H2C(5)H3, were studied by gas electron diffraction (GED). Vibrational amplitudes and shrinkage corrections were calculated on the basis of a normal coordinate analysis of the vibrational spectra measured in the vapor and liquid phases. The geometries of six possible conformers were optimized by ab initio HF/4-21G*/3-3-21G* calculations and the results were used in the data analysis as structural constraints. At 25-degrees-C, two conformers were observed. On the basis of the GED data analysis it is possible to identify the trans-trans conformer (TT, dihedral angles phi1(C4N1C2C3) = -phi2(C2N1C4C5) = 175(1)-degrees) as the most stable one, but the nature of the minor component is uncertain, since various mixtures of TT with one of three different gauche forms, TG, TG' or GG', fit the experimental data reasonably well. The best fitting model is a mixture of TT and TG (dihedral angles phi1 = 80(6)-degrees and phi2 = - 175(1)-degrees), with a relative abundance of 68(8) and 32%, respectively. The vibrational spectra are consistent with the existence of two conformers in the vapor and liquid phases. The structural parameters of TT determined by this analysis (with error limits of 3sigma and average values in brackets) are r(g)(N-C) = 1.471(2) angstrom, r(g)(N-Cl) = 1.765(2) angstrom, r(g)(C-C) = 1.530(3) angstrom, [r(g)(C-H)] = 1.118(3) angstrom, angle (alpha)NCC = 112.6(4)-degrees, [angle (alpha)NCH] = 109.6(18)-degrees, angle (alpha)CNC = 110.6(8)-degrees, angle (alpha)CNCl = 108.3(2)-degrees, [angle (alpha)CCH] = 109.4(11)-degrees and phi1 = (-phi2) = 175(1)-degrees. The structural differences between the TT and TG conformers were taken from the calculated ones.