MOLECULAR-STRUCTURES AND CONFORMATIONS OF DIETHYLAMINE AND TRIETHYLAMINE AS DETERMINED BY GAS ELECTRON-DIFFRACTION, ABINITIO CALCULATIONS, AND VIBRATIONAL SPECTROSCOPY

We have investigated molecular structures and conformations of diethylamine (DEA), C(3)H3C(2)H2N(1)HC(4)H2C(5)H3, and triethylamine (TEA), C(3)H3C(2)H2N(1)[C(6)H2C(7)H3]C(4)H2C(5)H3, by gas electron diffraction (GED) with the aid of ab initio calculations at the 4-21G level and vibrational spectroscopy. GED data of DEA have been reproduced by a mixture of TT, TG, and GG conformers (42 (16)%, 53 (24)%, and 5 (20)%) or that of TT, TG, and G'G conformers (42 (16)%, 58 (27)%, and 0 (19)%. In these compositions TT, GG, and G'G conformers have C4N1C2C3 and C2N1C4C5 dihedral angles of 175.3-degrees and 178.4-degrees, 69.7-degrees and 67.9-degrees, and -68.4-degrees and 98.2-degrees, respectively (dihedral angles are 180-degrees when the fragment takes the trans form). The TG conformer takes the values of 71 (12)degrees and 180 (18)degrees, and 71 (12)degrees and -179 (18)degrees for these dihedral angles in the two compositions, respectively. TEA has three conformers with C1, C(s), and C3 symmetry and their populations are 33 (43)%, 11 (41)%, and 56 (49)%, respectively. The C1 conformer has C4N1C2C3, C6N1C4C5, and C2N1C6C7 dihedral angles of -69.6-degrees, 72.9-degrees, and 58.9-degrees, respectively, and the corresponding angles of the C3 conformer are 74 (7)degrees. The C(s) conformer has a symmetry plane which includes N1C2C3 fragment (C3 anti to the lone pair on nitrogen) and the C6N1C4C5 and C4N1C6C7 dihedral angles are 166.1-degrees and -166.1-degrees, respectively. The structural parameters (r(g) and angle(alpha) with 3-sigma in parentheses) of the TG conformer of DEA are <r(N-C)> = 1.463 (1) angstrom, <r(C-C)> = 1.529 (1) angstrom, <r(C-H)> = 1.114 (3) angstrom, angle CNC = 113.7-degrees, <angle NCC> = 111.8 (6)degrees, and <angle CCH> = 109.5 (8)degrees and those of the C3 conformer of TEA are r(N-C) = 1.466 (1) angstrom, r(C-C) = 1.528 (1) angstrom, <r(C-H)> = 1.113 (3) angstrom, angle CNC = 112.6 (26)degrees, angle NCC = 112.1 (17)degrees, and <angle CCH> = 109.8 (9)degrees, where < > denotes average values. The angle CNC of DEA and other independent structural parameters have been fixed at values estimated from the 4-21G calculations. The differences between the structural parameter values of conformers have been fixed at theoretical values for each compound.