Kinya Iijima

MAIN CONFORMER OF GASEOUS GLYCINE : MOLECULAR STRUCTURE AND ROTATIONAL BARRIER FROM ELECTRON DIFFRACTION DATA AND ROTATIONAL CONSTANTS

Abstract The molecular structure of the main conformer of gaseous glycine has been determined by the joint analysis of electron diffraction data and rotational constants, in conjunction with the results of an ab initio calculation. The glycine molecule has an unionized form in the gas phase. The molecular skeleton is planar and the amine and hydroxyl groups are in the anti-position with respect to each other. The molecular parameters obtained are: r g (NC)=1.469(6) A, r g (CC) = 1.532(4) A, r g (CO)=1.207(1) A, r g (CO)=1.357(2) A, ∠NCC=113.0(3)°, ∠CCO=125.0(3)° and ∠CCO=111.5(2)°.

The molecular structure of acetylacetone as studied by gas-phase electron diffraction

Abstract The molecular structure of the enol form of gaseous acetylacetone has been determined from electron diffraction data. The molecular skeleton is planar but it is not symmetric. The bond distances in the ring are r g (CO) = 1.243 ± 0.002 A, r g (CO) = 1.319 ± 0.003 A, r g (CC) = 1.382 ± 0.007, r g (CC) = 1.430 ± 0.008 A, r g (OH) = 1.049 ± 0.015 A and r g (O⋯O) = 2.512 ± 0.008 A. The hydrogen atom of the internal hydrogen bond lies 0.45 ± 0.08 A out of the molecular plane with the OHO angle of 137 ± 7°.

An electron diffraction study of gaseous α-alanine, NH2CHCH3CO2H

Abstract The molecular structure of α-alanine has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument at UMIST. Only one conformer exists in the vapour of α-alanine and it is a neutral species. The principal structure,parameters of the molecule were observed to be: r g (C-N)=1.471(7) A, r g (C-C)=1.544(10) A, r g (C=O)=1.192(2) A, r g (C-O)=1.347(3) A, r g (C- m )=1.509(16) A, ∠NCC=110.1(8)°, ∠CC=O=125.6(7)°, ∠CC=0110.3(7)°, ∠CCC m 111.6(11)°, ∠NCC m =111.0(16)° and =l7(2)°,where is the dihedral angle between the NCC plane and the CC=O plane. The distortion is in the direction of reducing the repulsion between the methyl group and the carboxyl group.

Internal rotation of trifluoromethyl groups in hexafluoroacetylacetone

The potential energy barrier of internal rotation of the trifluoromethyl group is somewhat higher than that of the methyl group: the barrier height is 6.0 kcal mol-’ in (CF3)20 [l], 5.8 kcal mol-l in ClSCF, [2] and 5.2 kcal mol-’ in CF,SCF, [ 21. However, there are some compounds with comparatively low barrier heights such as 1.67 kcal mol-l in CFsCOF [3] and 1.19 kcal mol-l in CF, COCHzBr [ 41. Andreassen et al. [ 51 have determined the molecular structure of gaseous hexafluoroacetylacetone from electron diffraction data on the assumption of C, symmetry. They treated the internal rotation of the CF, groups as a rigid rotor and obtained the OCCF dihedral angle of 47’) although they mentioned that this is an “average structure” value. The present study was undertaken to investigate the rotational barrier height of CF, groups in hexafluoroacetylacetone by gas-phase electron diffraction. The molecular symmetry was checked again because acetylacetone has been revealed not to be symmetric [ 6,7]. Long and short camera-distance photographs were taken at room temperature by the use of an r3-sector. The exposure times were 50 and 95 s for the long and short camera-distance photographs, respectively, with an electron beam current of 0.6 ,uA. The wavelength of the electron beam was estimated from the measured accelerating voltage of 40 kV [ 81. The procedures of data reduction and molecular-structure analysis were the same as those used previously for acetylacetone [ 71. The vibrational mean amplitudes and the shrinkage corrections used in the analysis were calculated from the force field [91. Assuming an unsymmetric ring skeleton the analysis did not achieve convergence, as in the previous study [5]. The molecular skeleton was then assumed to be symmetric. The analysis assuming the rigid rotor to the CF, groups

Molecular structures of dipivaloylmethane complexes of lanthanides of samarium to holmiun as determined by gas electron diffraction

Abstract The molecular structures of six Ln(dpm) 3 (Ln = Sm, Eu, Gd, Tb, Dy and Ho; dpm = (CH 3 ) 3 CCOCHCOC(CH 3 ) 3 ) have been determined by gas-phase eletron diffraction. The experimental data are consistent with a trigonal prismatic structure with the slightly pitched OLnO planes. The observed LnO distances show the predicted lanthanide contraction: r a (SmO) = 2.278(7) A to r a (HoO) = 2.226(8) A. The OLnO and the pitch angles also depend on the atomic number. The structural parameters of the dpm ligand are as follows (average values with stranded deviations): r a (CO) = 1.279(8) A, r a (CC r ) = 1.411(12) A, r a (CC t ) = 1.523(19) A, r a (C t C m = 1.548(9) A, r a (O…O) = 2.716(23)A, ∠OCC r = 124.5(6)°, ∠CC r C′ = 122.2(5)° and ∠C r CC t = 117.6(9)°.

Molecular structure of tris(dipivaloylmethanato)-praseodymium (III) as studied by gas electron diffraction

Abstract The molecular structure of tris(dipivaloylmethanato)erbium has been determined by gas-phase electron diffraction. The experimental data are consistent with a monomeric trigonal prismatic structure. The structural parameters are as follows: ra(ErO) = 2.218 ± 0.005, ra(CO) = 1.274 ± 0.004, ra(CCring) = 1.405 ± 0.006, ra(CCtext) = 1.513 ± 0.014, ra(CCmeth) = 1.550 ± 0.006 A, and ∠OErO = 75.0 ± 0.5°. The folding of the ligand about the OO axis is 22.2 ± 1.5°, and the t-butyl groups rotate freely.

Molecular structure and internal rotation of (Z)-(propionaldehyde oxime) by gas-phase electron diffraction and microwave spectroscopy

Abstract The molecular structure of gaseous ( Z )-(propionaldehyde oxime) has been determined by the joint analysis of electron diffraction data and rotational constants. The molecular structure optimization has also been performed by molecular orbital calculations. The molecular skeleton is planar, but the internal rotation around the C-C(N) bond is a large amplitude motion with a potential barrier height of V 1 = 1.5 kcal mol −1 . The geometrical molecular parameters obtained are: r g (C m C) = 1.549 ± 0.003 A, r g (CC) = 1.497 ± 0.002 A, r g (C N) = 1.295 ± 0.001 A, r g (NO) = 1.430 ± 0.002 A, ∠ CCC = 109.4 ± 0.4°, ∠ CCN = 127.5 ± 0.3° and ∠ CNO = 109.3 ± 0.3°.

Reinvestigation of molecular structure and conformation of gaseous l-alanine by joint analysis using electron diffraction data and rotational constants

Abstract The molecular structure and conformation of gaseous l -form of α-alanine have been reinvestigated using electron diffraction data and rotational constants. It was reconfirmed that the molecules take a neutral form in the gas phase. The molecular parameters of two species were determined. In the vibrational ground state, the main species has the conformation with intramolecular hydrogen bonding of the NH 2 ⋯O C with the dihedral angle NCC O of −16.6(4)°, and the minor species has the conformation with intramolecular hydrogen bonding of the H 2 N⋯HO with the dihedral angle NCC–O of −16.9(4)°. Both species of gaseous l -alanine would take these minus dihedral angles in order to reduce the repulsion between the other atomic groups.

Molecular structure of gaseous acetoxime determined by electron diffraction

Abstract The molecular structure of gaseous acetoxime has been determined from a least-squares analysis of electron diffraction data. The skeleton of the gaseous molecule is planar and there are asymmetrical distortions on the CC distances and the CCN angles. The conformations of the two methyl groups are also different. The geometrical molecular parameters obtained are r g ( C 1  C 2 ) = 1.490(3) A , r g ( C 2  C 3 ) = 1.521(5) A , r g ( C 2  N ) = 1.289(1) A , r g ( NO ) = 1.423(2) A , ∠C1C2N = 116.4(2)°, ∠C1C2N = 123.4(3)° and ∠C2NO = 111.2(2)°.

Molecular structure and internal rotation of (Z)-chloroacetaldehyde oxime by gas-phase electron diffraction

Abstract The molecular structure of gaseous (Z)-chloroacetaldehyde oxime has been determined by gas-phase electron diffraction and molecular structure optimization has been carried out by molecular orbital calculations. The molecular skeleton is planar and the potential barrier height to internal rotation around the C C bond ( V 1 ) was estimated to be 2.7 kcal mol −1 . The geometrical parameters obtained are: r g ( Cl)=1.789±0.001A, r g (C C)=1.513±0.003A, r g (C N)=1.284±0.001A, r g (N O)=1.416±0.001A, ∠CCCl=109.7±0.2°, ∠CCN=124.9±0.3°, ∠CNO=110.6±0.2°.