J. Phys. Chem. 1992,96,4389-4396
4389
Molecular Structures and Conformations of Diethylamine and Triethylamine As Determined by Gas Electron Diffraction, ab Initio Calculations, and Vlbratlonal Spectroscopy Hiroshi Takeuchi, Tom Kojima, Tom Egawa, and Shigehiro Konaka* Department of Chemistry. Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: November 13, 1991; I n Final Form: February 12, 1992)
We have investigated molecular structures and conformations of diethylamine (DEA), C(3)H3C(2)HzN(1)HC(4)HzC(5)H,, and triethylamine (TEA), C(3)H,C(2)H,N(l) [C(~)HZC(~)H,IC(~)H,C(~)H~, 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 C4NlC2C3and CzNIC4CSdihedral angles of 175.3' and 178.4', 69.7' and 67.9', and -68.4' and 98.2', respectively (dihedral angles are 180' when the fragment takes the trans form). The TG conformer takes the values of 71 (12)' and 180 (18)', and 71 (12)' and -179 (18)' for these dihedral angles in the two compositions, respectively. TEA has three conformers with Cl, C,, and C, symmetry and their populations are 33 (43)%, 11 (41)%, and 56 (49)%, respectively. The Cl conformer has C4NlCzC3,C6NIC4Cs,and C2N,C6C7dihedral angles of -69.6', 72.9', and 58.9', respectively, and the corresponding angles of the C3conformer are 74 (7)'. The C, conformer has a symmetry plane which includes NIC& fragment (C3 anti to the lone pair on nitrogen) and the C6NIC4CS and C4NIC6C7dihedral angles are 166.1' and -166.1', respectively. The structural parameters (rgand L, with 30. in parentheses) of the TG conformer of DEA are (r(N-C)) = 1.463 (1) A, (r(C-C)) = 1.529 (1) A, (r(C-H)) = 1.114 (3) A, LCNC = 113.7', (LNCC) = 111.8 (6)', and (LCCH) = 109.5 (8)' and those of the C3conformer of TEA are r(N-C) = 1.466 (1) A, r(C-C) = 1.528 (1) A, (r(C-H)) = 1.113 (3) A, LCNC = 112.6 (26)', LNCC = 112.1 (17)', and (LCCH) = 109.8 (9)', where ( ) denotes average values. The LCNC 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.
Introduction
The conformationsof diethylamine (DEA) and triethylamine (TEA) have been studied by spectroscopic methods and theoretical calculations but the conformational behavior of these amines is not well elucidated. Figures 1 and 2 show some possible conformers of DEA and TEA, respectively. On the basis of the temperature dependence of IR spectra and depolarization ratio of a Raman spectrum of DEA, Verma' reported that at least TT and TG (and/or TG') conformers are present in the vapor, liquid, and various solutions whereas the TG or TG' conformer (preferably the TG) exists in the solid. They suggested that the TT conformer is predominant in fluid phase but that hydrogen bonding stabilizes the TG conformer in the solid phase. From the temperature dependence of the band intensities of the N-H stretching vibration, Wolff and Gamerz found that the 'IT conformer is more stable than the TG or TG' conformer with the enthalpy difference of 710 cal mol-' in n-hexane solution. The ab initio calculation carried out by Cox et ala3at the STO-3G level shows that the TT conformer is more stable than the TG conformer by 1.8 kcal mol-'. Conformation of TEA is more complex than that of DEA because of an additional N-C internal rotation. Molecular mechanics (MM2) calculations4revealed seven energy minimaSsq6 The three conformers shown in Figure 2 are more stable than the other conformers by at least 2.7 kcal mol-'. These conformers have C,, C,, and Cl symmetry. Bushweller et a1.6 investigated the conformation by dynamic NMR spectroscopy. The lH NMR spectrum in CBrF3 solution decomposed into two subspectra at (1) Verma, A. L. Spectrochim. Acta 1971, 27A, 2433. (2) Wolff, H.; Gamer, G. Spectrochim. Acra 1972, 28A, 2121. (3) Cox, R. H.; Kao, J.; Secor, H. V.; Seeman, J. I. J . Mol. Srrucr. 1986, 140, 93. (4) (a) Allinger, N. L. J . Am. Chem. SOC.1977, 99, 8127. (b) Profeta, S.,Jr.; Allinger, N. L. J . Am. Chem. SOC.1987, 107, 1907. (5) Fleischman, S.H.; Weltin, E.E.; Bushweller, C. H. J. Comput. Chem. 1985, 6, 249.
(6) Bushweller, C. H.; Fleischman, S. H.; Grady, G. L.; McGoff, P.; Rithner, C. D.; Whalon, M. R.; Brennan, J. G.; Marcantonio, R. P.; Domingue, R..P. J. Am. Chem. SOC.1982, 104, 6224.
0022-3654/92/2096-4389S03.00/0
107 K. The major subspectrum with relative area of 94% was assigned to the C, and/or Cl conformers and the minor to the C, conformer. It was impossible to discriminate between the C, and Cl conformers by dynamic NMR because of rapid conversion between them. Kumar' and Crocker and Goggins carried out conformational analysis by IR and Raman spectroscopy. They restricted stable conformers to the above three conformers by considering steric effects between ethyl groups. Kumar' concluded that the C3and C, conformers exist in the liquid and solid phases. No vibrational bands of the C1conformer could be observed. In the vapor phase the C, conformer is predominant and the existence of the CIand C, conformers was not definite. Crocker and Goggins reported that the conformer in the solid phase has C, symmetry and that the C, or C1conformer coexists with the C, conformer in the liquid. Thus the relative stability of these conformers remains ambiguous. DEA and TEA are fundamental amines which have conformations interconvertible by rotation about N - C bonds. Conformational analysis of these compounds is a key to understand conformational behavior of general alkylamines. Therefore, in the present study we have studied these molecules by gas electron diffraction (GED). The molecular structures and conformations of DEA and TEA are so complicated that they cannot be precisely determined by GED alone. In order to obtain supplementary information, we have performed ab initio geometry optimization and normal-coordinate calculations. The former gives reliable information on the differences among structural parameters of similar types and the latter gives mean amplitudes and shrinkage corrections. These quantities have been used in the present GED analysis. Molecular mechanics calculation has widely been used as a convenient tool for conformational analysis? The structural data obtained in the present study will be useful to check and improve the reliability of molecular mechanics calculations for amines. No (7) Kumar, K. Clrem. Phys. Lett. 1971, 9, 504. (8) Crocker, C.; Goggin, P. L.J . Chem. Soc., Dalton Trans. 1978, 388. (9) Burkert, U.; Alhnger, N. L. Molecukar Mechanics; American Chemical Society: Washington, DC, 1982.
0 1992 American Chemical Society
Takeuchi et al.
4390 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
TABLE II: Dibednl Angles 9 (deg),Relative Emrgies AE (Lad mol-'), and Populations X (5%) Given by the MM2 and the 4 2 1 6 Calculations Diethylamine" TT
TG'
MM2
TG
61 180 -66 63 59 -62 97
TT
TG'
Gt
GG
GG
TG GG G'G GG'
67
180 -176 175 60 97 -60
4-21G
AJ9 0.0 1.19 1.08 2.03 4.00 4.10
xb 60 16 20 4 0 0
91 175 -68 69 70 -68 103
9, 178 -176 178 68 98 -71
MM2
41
CI -67 C, C,
Q
Figure 2. Some possible conformers of triethylamine. TABLE I: Exwrimentd Conditions diethylamine 26 245.1 0.06358 38-42 8-10 26-27 2.3-2.4 0.05 4
xb 42 35 15 6 2 0
Triethylamined
Figure 1. Six possible conformers of diethylamine.
room temperature, OC camera distance, mm electron wavelength, A sample pressure, Torr background pressure, 10" Torr exposure time, s beam current, rA uncertainty in the scale factor, % no. of plates used
hEc 0.0 0.51 1.02 1.59 2.40 3.07
triethylamine 29 245.1 0.06360 34-38 12-15 23-27 1.8-2.1 0.05 4
calculations have been performed for DEA and no detailed structural information has been reported for TEA. Thus calculations have been performed with the MM2 force field4 and compared with experimental results.
Experimental Section The samples of DEA and TEA, with purities higher than 99% were obtained from Tokyo Chemical Industry Co., Ltd. An apparatus equipped with an r3 sectorlo was used to record the diffraction patterns of DEA and TEA on 8 X 8 in. Kodak electron image plates at room temperature. The diffraction patterns of carbon disulfide were recorded after those of DEA or TEA in order to determine the wavelength of incident electrons by using the value r,(C-S) = 1.5570 A." Other experimentalconditions are listed in Table I. Optical densities were measured with an interval of 100 r m by using a microphotometer of the doublebeam autobalanced type. Five adjacent points were averaged and the resulting data interval was 0.5 mm. After optical densities were converted to total intensities, leveled intensities were obtained by dividing the total intensities by theoretical backgrounds. Elastic and inelastic scattering factors were taken from refs 12 and 13, respectively. (IO) Konaka, S.; Kimura, M. Preacnted at the Austin Symposium on Gas Phase Molecular Structure, Austin, Texas, 1990;S2I. (11) Tsuboyama, A.; Murayama, A.; Konaka, S.;Kimura, M. J . Mol. Struct. 1984, 118, 351.
-63 56
42
63 176 56
93 54 56 56
4-21G
AE
xb
0.22 0.01 0.0
45 33 22
91 -70 -67 76
92 73 166 76
93 59 59 76
me xb 0.0 0.20 0.24
64 22 14
and q52 denote the dihedral angles of C4NIC2C3 and C2NIC4CS, respectively. The geometry optimization for TT, GG', and G'G conformers is made without restriction of C, symmetry. *Calculated at room temperature. CTotal energies of these conformers are -211.861 40, -211.86058, -211.85977, -211.85887, -211.85651, and -211.85765 au, respectively. 92,and 9, denote the dihedral angles of C4NIC2C3, C6N1C4CS, and C2NIC6C7, respectively. The geometry optimization of the C, and C3 conformers was performed without restriction of symmetry. eTotal energies of CI, C,, and C3conformers are -289.761 72, -289.761 41, and -289.761 34 au, respectiveIY.
The IR spectrum in the vapor of DEA was measured with a Bomem Model DA3.16 spectrometer by using a IO-cm cell with KBr windows. The IR spectra of TEA in the vapor and liquid were measured with Digilab Model FTS-14B and FTS-65DF Fourier transform spectrometers, respectively. The Raman spectrum of TEA in the liquid was measured with a Spex Model RAMALOG-10 spectrometer using an A r' laser (514.5 nm). A Jasco Model R300S Raman spectrometer was used to measure the Raman spectrum of DEA in the liquid using a He-Ne laser (632.8 nm). The observed frequencies of DEA and TEA are listed in Tables SI and SI1 (supplementary material), respectively.
Theoretical Calculations Molecular mechanics calculations were performed by using the MM2 force field.4 The theoretical dihedral angles, CNCC, and energies are listed in Table 11. Ab initio calculations were carried out by the use of the 4-21G basis setI4and the program TEXAS.'$ We optimized the geometries of six conformers of DEA shown in Figure 1. The largest residual Cartesian force in final geometries was less than 0.00073 au. Relative energies and structural parameters are presented in Tables I1 and 111, respectively. The number of possible conformers of TEA is larger than that of DEA because of the increase of rotational degrees of freedom. By referring to the MM2 results, the 4-21G calculations were restricted to the three conformers shown in Figure 2. Resulting residual Cartesian forces were less than 0.00086 au. Conformational energies and structures are summarized in Tables I1 and IV, respectively. All the CCNCC fragments of the C, conformer take the same TG form, whereas the C1and C, conformers have two TG and one GG, and one TT and two GG conformations, respectively. Thus, neither the GG' nor the G'G conformation exists in these three conformers of TEA. According to the MM2 calculations,6 the conformers with the CCNCC fragments of GG' (12) Kimura, M.; Konaka, S.;Ogasawara, M. J . Chem. Phys. 1%7,46, 2599. (13) Tavard, C.; Nicolas, D.; Rouault, M. J . Chim. Phys. 1967.64540. (14)Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J . Am. Chrm. Soc. 1979, 101, 2550. (15) F'ulay, P. Theor. Chim. Acta 1979, SO, 299.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4391
Structures of Di- and Triethylamine TABLE IIk 421C Geometries of Dietbylami& parameter TT TG’ TG GG 1.472 1.473 1.476 1.476 1.472 1.473 1.474 1.474 1.005 1.003 1.004 1.003 1.535 1.544 1.537 1.536 1.091 1.083 1.083 1.083 1.083 1.083 1.091 1.088 1.084 1.082 1.081 1.081 1.083 1.084 1.083 1.084 1.081 1.084 1.082 1.082 1.535 1.535 1.536 1.542 1.083 1.083 1.080 1.083 1.091 1.089 1.091 1.081 1.081 1.081 1.081 1.085 1.083 1.083 1.083 1.083 1.084 1.084 1.085 1.082 115.2 116.8 116.2 118.1 111.4 112.0 111.4 112.2 111.5 111.9 111.2 111.6 110.0 115.3 111.6 111.4 112.4 107.5 107.3 106.7 107.7 107.6 112.2 113.1 110.0 109.7 108.6 108.6 109.6 109.3 109.6 109.7 110.7 111.4 109.4 109.3 110.9 110.4 110.3 110.3 109.6 110.4 111.5 111.7 110.1 109.7 109.8 114.9 107.6 107.4 108.2 107.2 112.5 113.3 112.3 108.4 109.5 109.5 109.2 109.3 109.9 109.6 110.0 109.7 109.5 109.6 109.6 110.3 110.9 110.9 110.9 110.5 110.7 110.6 110.7 111.3 53.5 52.8 62.6 61.9 -177.4 -177.9 173.6 172.8 -66.6 -67.2 -56.8 -58.4 54.2 54.8 55.6 58.5 174.9 175.6 176.2 178.1 -65.1 -64.5 -63.9 -61.7 178.4 -175.7 178.2 67.9 -170.3 -62.7 -62.3 -56.8 61.5 55.5 -55.2 55.6 -162.1 -158.3 -56.4 62.5 69.1 69.7 175.3 -68.4 -172.1 -172.0 -61.8 54.3 169.4 -54.3 -54.3 56.0 50.2 53.5 49.3 -64.3
TABLE I
G’G
GG’
1.468 1.47 1 1.000 1.545 1.083 1.083 1.082 1.084 1.084 1.544 1.084 1.083 1.084 1.084 1.082 119.3 113.7 113.6 116.2 107.7 107.3 109.3 109.1 111.2 110.3 110.5 114.5 108.8 107.9 108.7 109.8 110.3 110.4 111.4 62.7 -177.4 -57.9 58.3 177.5 -61.9 98.2 52.4 169.9 70.1 -68.4 -137.0 -20.0 -40.3
1.478 1.473 1.Ooo 1.538 1.084 1.088 1.082 1.085 1.080 1.536 1.09 1 1.083 1.082 1.084 1.08 1 117.7 112.4 111.9 112.0 108.4 110.5 108.6 110.2 109.5 110.7 111.0 112.2 112.0 107.O 109.3 108.7 111.3 110.2 109.6 57.1 177.1 -62.2 68.6 -171.9 -51.8 -7 1.O -140.0 -24.4 -124.6 103.2 54.6 169.3 156.6
“Bond lengths in angstroms and angles in degrees.
or G’G form are less stable than the C1conformer by 2.9 kcal mol-’. This is consistent with the result that the GG’and G’G conformers of DEA are less stable (Table 11).
Normal-Coordinate Analysis Dktbylpmiw. Normal-coordinate analysis was carried out with our programs NVLS and N V M A which enable us to refine the constants of general valence force field in the least-squares fitting of theoretical frequencies to observed on= and to calculate mean amplitudes and shrinkage corrections,I6 respectively. The force constants for internal coordinates except the torsional ones were taken from diethyl ether” and dimethylamine.’* The local symmetry coordinates in ref 19 and torsional coordinates given by Hilderbrandt” were used. They were refined so as to reduce the difference between observed and calculated frequencies. The assignment reported in the literature’ was modified as shown in Table SI. The torsional force constant of methyl groups was evaluated to be 0.097 mdyn A rad-? so as to reproduce the fre(16) Hargittai, I. In Stereochemical Application of Gas Phase Electron VCH ; Publisher: New Dflraction, Purr A; Hargittai, I., Hargittai, M.,as. York, 1988; Chapter 1. (17) Wieser, H.; bidlaw, W. 0.; Krueger, P. J.; Fuhrer, H. Spectrochim. Acta 1968, 24A, 1055. (18) Dellepiane. G.; Zerbi, G.J . Chem. Phys. 1968, 48, 3573. (19) Hamada, Y.;Hashiguchi, K.; Hirakawa. A. Y.;Tsuboi, M.;Nakata, M.; Tasu@, M.;Kato, S.;Morokuma, K. J . Mol. Specrrosc. 1983,102, 123. (20) Hilderbrandt, R. L. J . Mol. Specrrosc. 1972, 44, 599.
V 4-216 Geometries of Triethylamine“
parameter
CI 1.476 1.478 1.477 1.545 1.080 1.083 1.084 1.083 1.082 1.538 1.090 1.083 1.084 1.08 1 1.08 1 1.538 1.090 1.080 1.083 1.OS 1 1.082 113.9 115.4 113.5 116.0 108.2 107.0 109.4 109.2 109.8 111.1 111.4 112.0 112.1 107.3 109.9 108.2 110.0 109.2 112.0 112.8 111.8 107.2 110.0 107.9 109.8 109.4 112.0 178.1 58.6 -62.2 171.8 52.0 -68.4 169.8 50.3 -70.5 -1 52.4 58.9 83.6 -33.8 -65.6 177.6 -69.6 -167.1 167.1 52.4 68.5 -48.4 64.2 72.9 -59.1 -173.7 -51.1 -168.5
C, 1.476 1.477 1.477 1.545 1.080 1.080 1.084 1.082 1.083 1.538 1.083 1.089 1.084 1.082 1.08 1 1.538 1.OS9 1.083 1.083 1.08 1 1.082 115.5 115.5 112.3 116.0 107.7 107.7 109.3 109.3 109.7 111.4 111.4 112.8 106.6 112.3 108.2 109.7 109.8 112.1 109.7 112.8 112.3 106.5 109.7 108.2 109.8 109.6 112.1 180.0 60.5 -60.5 -168.8 71.5 -49.5 168.8 49.5 -71.5 -58.5 58.6 -177.1 66.1 -66.0 177.2 -67.0 -166.0 170.2 55.8 69.4 -47.3 67.0 166.1 -55.8 -170.2 47.5 -69.3
‘Bond lengths in angstroms and angles in degrees.
c3 1.479 1.479 1.479 1.537 1.091 1.080 1.084 1.08 1 1.082 1.537 1.09 1 1.080 1.084 1.081 1.082 1.537 1.09 1 1.080 1.084 1.081 1.082 113.4 113.4 113.3 111.8 111.4 107.9 110.3 108.2 110.2 109.2 111.9 111.9 11 1.3 107.9 110.5 108.1 110.3 109.2 111.9 11 1.9 111.3 107.9 110.4 108.2 110.3 109.2 111.9 175.0 55.2 -64.8 175.5 55.6 -64.3 175.4 55.5 -64.5 -1 52.8 76.1 83.0 -34.0 -47.9 -165.0 75.7 -152.9 -48.3 -165.5 83.1 -34.0 -153.4 76.2 82.6 -34.5 -48.0 -165.0
4392 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Takeuchi et al.
TABLE V Mean A m n h d e s ( I ) and Interatomic Distances (r.) for Diethylamine TT TG atom pair p1cd pled low r. PW 1.031 0.073 0.073 0.074) (3) 0.079 %$3) 1.109 0.079 0.079 1.458 0.051 0.048 0.048 0.05 1 1.458 0.05 1 0.05 1 1.525 0.056 0.053 0.053 0.056 1.525 0.056 0.053 0.053 0.056 2.111 0.102 O' 02} (6) 0.102 0.109 0.109 ::t:i}(6) 2.158 0.109 2.418 0.070 0.070 0.070 0.070 2.455 0.070 0.070 2.456 0.07 1 0.071 0.07 1 0.070 3.754 0.072 (1 3) 0.077 0.077 3.752 0.153 0.071 0.158 (45) 0.076 0.156 0.084 (35) 4.885 0.146 (35) 0.094
~~
GG r.
1.030 1.108 1.462 1.460 1.527 1.526 2.112 2.158 2.440 2.482 2.448 3.763 3.073 4.405
I='* 0.073 0.079 0.05 1 0.05 1 0.056 0.056 0.102 0.108 0.069 0.070 0.070 0.171 0.151 0.279
PW
r.
0'074)(3) 0.079 0.048 0.053 0.053 0.070 0.070) (4) 0.070 0.177)(45) 0.156 0.274 (13)
1.108 1.029 1.462 1.460 1.526 1.532 2.094 2.161 2.468 2.476 2.530 3.083 3.131 3.924
OThe mean amplitudes for relatively important atom pairs are listed. Numbers in parentheses are 3 times the standard errors. bAverage value. 'Geminal atom pairs. TABLE VI: Mean Amplitudes ( I ) and Interatomic Distances (r,) for Triethylamine (A).
c,
Cl
atom pair
I"'"' 0.079 0.050 0.050 0.050 0.054 0.054 0.054 0.105 0.108 0.067 0.068 0.067 0.065 0.065 0.065 0.131 0.140 0.137 0.136 0.078 0.261 0.072 0.132 0.150
pbd
0.079 (3) 0.05 1
0.054 0.054 0.079 0.080
0.077 0.120 0.128 0.125 0.125 0.093 0.275 0.087 0.167
ra
1.106 1.462 1.464 1.463 1.535 1.528 1.528 2.098 2.163 2.538 2.476 2.487 2.438 2.458 2.433 2.978 3.1 14 3.065 3.068 3.739 3.651 3.783 4.590 4.337
c 3
pkd
10"
ra
PI*
0.079 0.050 0.050
0.079 (3) 0.05 1 0.051
1.106 1.462 1.463
0.079 0.050
0.079 (3) 0.05 1
1.107 1.465
0.054 0.054
0.054 0.054
1.535 1.528
0.054
0.054
1.527
0.105 0.108 0.068
0.1 10 (6) O'lo7)
0.079 0.079
2.094 2.163 2.538 2.488
2.107 2.162 2.475
0.065
0.077 ) ( W
2.459
0.065
0.077
0.065
0.077
2.416 0.136
0.124 (13)
3.081
0.138
0.127
0.133
0.122
3.006
0.078
0.093 (14)
3.742
3.764 3.764 4.913
0.170
0.187 (48)
4.200
0.105 0.108 0.067 0.067
0.075 0.246 0.101
::3 (6)
)
PU
0.080
)
ra
(12) 2.434
3.104 (13)
::;;;}
(14) 0.118 (48)
"he mean amplitudes for relatively important atom pairs are listed. Numbers in parentheses are 3 times the standard errors. bAverage value. 'Geminal atom pairs.
quency of 250 cm-I.' The torsional force constant of ethyl groups was estimated from the potential function determined for ethylmethylamine (0.095 mdyn A rad-2).21 The final force constants are listed in Table SI11 (supplementary material). Triethylamine. The torsional force constant of ethyl groups was estimated to be 0.138 mdyn A rad-2 from the potential barrier for the torsion about the N - C bonds in trimethylamine.22 Other force constants were transferred from DEA and trimethylamine.'* Calculated frequencies and assignments,and force constants are listed in Tables SI1 and SIII, respectively.
Structural Analysis and Results Assumptions on structural parameters and vibrational amplitudes were made by referring to the results of the 4-21G and normal-coordinate calculations, respectively. The following constraints on the structures were adopted. (1) The differences in N-C and C-C bond lengths were fixed at the differences in the rg distances which were derived from the re(4-21G) structure and empirical corrections, r - re(4-21G) (-0.012 A for r(N-C) and -0.008 A for r(C-C)k.23 (21) Durig, J. R.; Compton, D. A. C. J . Phys. Chem. 1979, 83, 2873. ( 2 2 ) Rinehart, E. A,; Reinhart, P. B.; Wollrab, J. E. J . Mol. Specirosc. 1973, 47, 556.
(2) The CNC bond angles of TEA were refined in one group. Similarly the C-H bond lengths, and NCC and CCH bond angles of DEA and TEA, were refined in groups. The differences of structural parameters in each group were kept constant at the 4-21G values (see assumption 5 for LCNC of DEA). (3) NCH bond angles and NCCH dihedral angles were fixed at the 4-21G values. (4) Some CNCC dihedral angles were refined and the others were fixed at the 4-21G values. In the case of TEA, the choice of the dihedral angles to specify the conformation is somewhat arbitrary. For example, either C4NlC2C3or C6NlC2C3may be taken as the dihedral angle around the NI-C2 bond. For the C1 conformer, dihedral angles C4N1CzC3,C6N1C4CS, and C2N1C6C7 were constrained to be -69.6', 72.9O, and 58.9O, respectively. The C6NlC4C5and C4N1C6C7angles of the C, conformer were fixed at 166.1O and -166.1°, respectively, and the C3C2NlLpangle was fixed at 180°, where Lp denotes the lone pair on nitrogen. (5) The r(N-H), LCNH, and LCNC values of DEA were estimated from the rJ4-2 1G) values and the empirical correct i o n ~ .The ~ ~ corrections for r(N-H), LCNH, and LCNC were taken to be 0.031 A, -3.4O, and -2S0 based on the rg- re(4-21G) (23) (a) Schlfer, L. J . Mol. Srruci. 1983,100,51. (b) SchBfer, L.; Van Alsenoy, C.; Scarsdale, .I. N. J. Mol. Siruci. (THEOCHEM) 1982, 86, 349.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4393
Structures of Di- and Triethylamine TABLE VI1 Results of Conformational Analysis for diethyl amine^ population, % model TT TG’ TG GG 1 42 (16) 53 (24) 5 (20) 2 42 (16) 58 (27) 25 (28) 3 63 (14j 39 (14) 4 61 (14) 5 47 (19) 53 (19) 6 80 (12)
G’G
GG’
0 (19) 12 (21, 20 (12)
91b 71 (12) 71 (12) . .
926
Rc
180 (1 8) -179 (18) . ,
71 (7)d -66 (12) 178 (32)‘
69 (7)d 178 (23) -179 (32)‘
0.059 0.060 0.066 0.067 0.072 0.079
a,+l and b2 are in degrees and R is dimensionless. Numbers in parentheses are 3 times the standard errors given by the least-squares refinement. The parameter values in models 1 and 2, which are in agreement within the estimated limits of error, should be regarded as the final results of the present study. and & denote C4N1C2C3and C2NIC4C5dihedral angles, respectively, of the TG (models 1 and 2), GG (model 4), TG’ (model 5) = $q - 1.8 was used. ‘The constraint of & = 6, + 3.1 was used. or TT (model 6) conformer. < Rfactor. dThe constraint of
-0.5
I
0
1
v
-
_
3
4
5
I
6
r/A Figure 3. Experimental radial distribution curves (0)and the theoretical ones (-) for six conformers of diethylamine. Relatively important atom pairs are indicated by vertical bars.
value23for amines, the r, - re(4-21G) value23for dimethylamine, and the r, - r,(4-2 1G) value23for dimethylamine, respectively. Mean amplitudes were refined in groups. Groups were separated at 1.3, 1.8,2.25,2.8, 3.5, and 4.0 A by referring to the peaks of observed radial distribution (RD) curves (see Figures 3 and 6). The differences in the mean amplitudes of the same group were fixed at the calculated values, which are listed in Tables V and VI for DEA and TEA, respectively. Adjustable structural parameters, mean amplitudes, and index of resolution were refined by the least-squares calculations on the molecular scattering intensity. Shrinkage corrections were fixed at calculated values. Asymmetry parameters were estimated by the conventional methods.24 The details of conformational analysis of each compound are described below. The potential barriers for internal rotation about N - C bonds for DEA and TEA were estimated to be about 3 and 4 kcal mol-I, respectively, by referring to those of dimethylamineZ1and trimethylamine.22 Since these values are not very low, we refined the mean amplitudes in the small-amplitude approximation. The refined values were considered to include the effect of the large-amplitude motion. Since there are no large differences between calculated and observed mean amplitudes as shown later, the approximation is expected to be good representation for the N-C torsion. Diethylamine. Prior to the conformational analysis, the least-squares analyses based on the single conformer model were carried out in which each of six conformers exists with abundance (24) (a) Kuchitsu, K. Bull. Chem. Soc. Jpn. 1 9 6 7 , 4 4 9 8 . (b) Kuchitsu,
K.; Bartell, L. S.J . Chem. Phys. 1961, 35, 1945.
V V V
-
AsM( s) .
Ar-----n------v--~
10
1
I
2
- -
v, y A
\/ “ V
W
0.1 0.0 -0.1
1
s/A-l
20
30
Figure 4. Experimental molecular scattering intensity (0)and the theoretical one (-) of diethylamine with the composition of 42% ‘IT 53% TG 5% GG; A M ( $ ) = ~M(.Y)”” -SM(S)~”.
+
+
of 100%. The resultant RD curves are displayed in Figure 3. These curves were calculated by using the final values of adjustable parameters. It should be noted that six different experimental curves in Figure 3 were obtained by making use of theoretical values of each conformer for the unobserved inner part of the experimental intensity curve. It is apparent that the ‘IT conformer is present since the peak observed at 4.8 A is peculiar to this conformer. Therefore, we made conformational analysis by assuming the TT conformer exists with other conformers. There are 3 1 possible combinations of the TT conformer and other five conformers. All of them were examined as different models. Populations were determined as adjustable parameters in the least-squares fitting of molecular scattering intensity. The results which give positive conformational populations are presented in Table VII. In models 1, 2 and 5 , two CNCC dihedral angles of the TG or TG‘ conformer were determined by GED. The CNCC dihedral angles of the GG conformer in model 4 were refined under the constraint of +(C2N1C4C5)= +(C4N1C2C3)- 1.8O by referring to the 4-21G calculationsand those of the lT conformer in model 6 were refined in a similar manner. The CNCC dihedral angles of the other conformers were fixed at the 4-21G values in these models. It was impossible to refine the CNCC dihedral angles in model 3. Observed dihedral angles are listed in Table VII. The R factors2sfor models 1 and 2 are smaller than those for the other models by 0.0064.019. Therefore, these models are considered to be more plausible than the other models. In models 1 and 2, the TT and TG conformers have similar populations and the population of the GG or G’G conformer is equal to zero within
-
( 2 5 ) R = l ~ i W ( ~ M ( s ) i ) 2 / ~ i ~ . ( s M ( swhere ) p ~ A.M(s), ) 2 ~ 1 ~ 2 sM, (s)pM - sM(s)plCdand W,is a diagonal element of the weight matrix.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
4394
Takeuchi et al. I
I
I
I
I
I
1
0
1
2
3
4
5
6
1
Diethylamine
-1
W
0
-
1
Af(r)
- -
I
1
I
1
I
I
1
2
3
4
5
6
r/A Figure 5. Experimental radial distribution curve (0)and the theoretical one (-) of diethylamine with the composition of 42% TT + 53% TG + 5% GG; AAr) =f(r)"" -I(r)-IC. Relatively important atom pairs of the TT conformer are indicated by vertical bars.
r/A Figure 6. Experimental radial distribution curves (0)and the theoretical ones (-) for C,,C,, and C3conformers of triethylamine. Relatively important atoms pairs are indicated by vertical bars.
TABLE VIII: Observed Structures of Diethylamine' r(N,-C2) r(Ni-C4) r(c2-c3) r(C4-G) (r(C-H))b r(N-H)
( LCCH ) ( LCNH)
@(C4NIC2C3) @(C2NIC4CS) population
TT 1.460
TG 1.464
GG 1.464
1.527 1.527 1.114 (3) 1.036c 112.7'
1.528 1.114 (3) 1.03Y 1 13.7'
1.534 1.113 (3) 1.034c 115.6c
109.6 (8) 108.1' 175.3d 178.4d 0.42 (16)
109.5 (8) 107.9' 71 (12) 180 (18) 0.53 (24)
109.6 (8) 108.5' 69.7d 67.9d 0.05 (20)
"The result of model 1. Bond lengths (r,) in angstroms and angles (r,) in degrees. Numbers in parentheses are 3 times the standard errors given by the least-squares analysis. Index of resolution is 0.88 (2). The parameter values of TT and TG conformers in model 2 are in agreement with those in model 1 within experimental uncertainties. bAverage value. CEstimated by adding the empirical correction to the 4-21G value. dFixed at the 4-21G value.
experimental uncertainties. There were no significant differences between the two models as for the structural parameters and mean amplitudes. Therefore, we present the results of models 1 and 2 as the final results. Molecular scattering intensities and RD curves of model 1 are shown in Figures 4 and 5, respectively. The values of experimental structural parameters and mean amplitudes are presented in Tables VI11 and V, respectively. Observed mean amplitudes are in agreement with the calculated ones within experimental uncertainties. Model 5 includes the TG' conformer, which has a large population according to the MM2 and 4-21G calculations. The R factor25of this model was 0.072, which is much larger than that of model 1 (0.059). Therefore, this model was rejected in the present data analysis. The NC& angle of the TG' conformer in model 5 (115.8 (7)') was 3.1' larger than that of the TG conformer in model 1 (1 12.7 (6)'). This difference is considered to be the main cause of the discrepancy between experimental and theoretical intensity data in model 5 . Triethylamine. Figure 6 shows the RD curves for individual conformers. It is apparent that no model with only one conformer can reproduce the observed curve. Therefore, a t least two conformers exist in the vapor. The best result was obtained for the
1.0 I
0.1 0.0 -0.1 -
1
I
I
I
I
V
'
-v
*-
"
1
I
ASM(S) WJ
-~-"~-~"--%/---~-+
4
1
I
1
1
1
I
(26) Bartell, L. S.; Kohl, D. A. J . Chem. Phys. 1963, 39, 3097.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4395
Structures of Di- and Triethylamine
I
I
I
- "-
0
'
I
I
Afb-1
_ _ -
-
v
I
I
I
I
I
I
I
I
3 4 5 6 r/A Figure 8. Experimental radial distribution curve (0)and the theoretical one (-) of triethylamine with the composition of 33% C, 11% C, + 56% C3;AAr) -f(r)C.L.Relatively important atom pairs of the C, conformer are indicated by vertical bars.
2
1
+
=foe*
TABLE I X Observed Structures of Triethylamine' C,
C,
C.
~
1.466
r(N1-G)
1.463
1.463
r(Nl-C4) r(NI-C6) r(C2-C3)
1465 1:464}(1) 1.536 1.529 1.529 1.112 (3) 113.11 114.6 (26) 112.7
1.529 1.529 1.112 (3)
w 4 - w r(c6-c7)
(r(C-H))b
LC~NIC~ LC2NIC6
fC,N,C,
ki"") HM::}
(17) 113.0 (L N C H ) 108.9' (LCCH)b 109.8 (9) +(C4NICZC3) -69.6' ~$(C~NIC~CS) 72.9' +(CzN IC&) 58.9' 0.33 (43) population
LN1C4CS
LN1c6c7
1.528 1.528 1.113 (3)
),.,,l
114.7 (26)
::::;} iip\B)
LN;C;C;
::%}
(1) 1.528 (l)
(17)
108.9' 109.8 (9) -65.4d 166.1' 61.6c 0.11 (41)
112.6 (26) 112.6
iiili)
(17)
109.6' 109.8 (9) 74 74 (7) 74 0.56 (49)
"Bond lengths ( r p ) in angstroms and angles (r,) in degrees. Numbers in parentheses are 3 times the standard errors given by the leastsquares analysis. Index of resolution is 0.89 (2). bAverage value. 'Fixed at the 4-21G value. dParameter dependent on the CNC angles. CParameterdependent on the C4NIC6C7 dihedral angle and CNC angles.
population of 60% (MM2) and 42% (4-21G) and that the TG conformer exists with the population of 20% (MM2) and 15% (4-21G) (Table 11). Therefore, the theoretical calculations reproduce the observed population of the most stable conformer, TT, but underestimate the population of the next stable conformer, TG. It is noted that the TG' conformer was not detected experimentally, in disagreement with the theoretical calculations which predict the population of 16% (MM2) and 35% (4-21G). As shown in Table SI, the vibrational bands observed in the liquid and vapor phases were interpreted by considering the existence of the TT and TG conformers. However, the assignment of the vibrational bands of 960-750 cm-' in the solid phase remains unsettled since they could not be assigned to a single conformer and some of them disappeared in the liquid and vapor phases. It seems necessary to reinvestigate solid-phasevibrational spectra. The observed conformationalcomposition of TEA is 56 (49)% C3 33 (43)% Cl 11 (41)% C,. The multiplicities of the C3, C,, and C, conformers are 2,6, and 3, respectively, and hence the C3conformer is the most stable. This conclusion is consistent with
+
+
vibrational spectroscopic data since the normal-coordinate calculations show that the C,, Cl, and C, conformers are present in the liquid and vapor phases and that the C3 conformer exists in the solid phase (Table SII). The populations of the three conformers given by the MM2 and 4-21G calculations are consistent with the observed ones within the experimentalerrors (Table 11). The CNCC dihedral angles determined for the TG conformer of DEA are consistent with the MM2 and 4-21G calculations. The CNCC dihedral angles of the TT conformer, which are fmed at the 4-21G values in the GED analysis, are in agreement with those by the MM2 calculations within 5 O . This consistency is not the case for the CNCC angles of TEA. The 4-21G value of the C4NIC2C3dihedral angle of the C3 conformer, 76O, is in good agreement with the experimental result, 74 (7)O. However, the value, 56O, given by the MM2 calculationsis much smaller. The CsNICICsdihedral angles of the CI and C, conformers by MM2 calculations (63O and 176') are also different from those assumed in the analysis (73' and 166O, 4-21G values). The GED analysis employing the MM2 dihedral angles was inconsistent with the experimental data; we obtained the composition of 79 (37)% C1 21 (37)% C, with an R factor of 0.089 by this analysis, whereas the R factor in the best analysis is 0.055. Therefore, the MM2 calculationsgive satisfactory results for the conformationalcomposition but fail to predict the CNCC dihedral angles. Finally, comments are added on the energies of the TG' and TG conformers and the dihedral angles of the GG' and G'G conformers of DEA. According to the 4-21G calculations on eth~lmethylamine,~' the energies of the G' and G conformers are higher than that of the T conformer by 0.4 and 0.9 kcal mol-', respectively. These values are similar to the energy difference between the TG' and TT conformers and that between the TG and TT conformers of DEA, respectively. According to the 3-21G (N*) and 6-3lG* calculations on ethylmethylamine,28,29 the G conformer is less stable than the T conformer by 1.1 and 0.9 kcal mol-', respectively, but the energy of the G' conformer is larger than that of the T conformer by 1.2 kcal mol-I. That is, the G' conformer is slightly less stable than the G conformer, contrary to the 4-21G calculations. Therefore, it is expected that the 4-21G relative energy for the TG' conformer of DEA is also underestimated. This explains why the 4-21G estimation for the population of the TG' conformer is quite different from the experimental result. The 4-21G calculations on DEA show that one of the ethyl groups in the G'G and GG' conformers rotates from the staggered position (60O) by about 40°, whereas the CNCC dihedral angles of the other conformers of DEA are within *60° f 10' or 180° & 5'. This difference can be ascribed to the steric repulsion between the ethyl groups which is larger in the GG' and G'G conformers than in the other conformers of DEA. Bond Lengths and Angles. The r,(N-C) of mathylamine30 (1.472 (3) A) is considerably longer than that of dimethylamine3I (1.456 (2) A) and trimethylamine3' (1.455 (2) A). A similar tendency is found for ethyl compounds. The N-C bond length of ethylamine32(1.470 and 1.475 (10) A) is longer than those of DEA (1.460-1.464 (1) A) and TEA (1.463-1.466 (1) A). The N-C distances of dimethylamine" and DEA are close to those of R3Nwhich are 1.455 (2) A, 1.463-1.466 (1) A, and 1.460 (5) 8,for R = Me," Et, and i-Pr," respectively. However, they are shorter than the N - C distance of diis~propylamine~~ (1.470 (4) A). The LCNC of dimethylamine3' and DEA (1 11.8 (6)O and 112.7-1 15.6') are considerably smaller than that of diiso-
+
(27) Van Alsenoy, C.; Scarsdale, J. N.; Williams, J. 0.; Schifer, L. J. Mol. Struct. (THEOCHEM) 1982, 86, 365. (28) Batista de Carvalho, L. A. E.; Teixeira-Dias, J. J. C.; Fausto, R. Struct. Chem. 1990, 1, 533. (29) Schmitz, J. R.; Allinger, N. L. J . Am. Chem. SOC.1990,112,8307. (30) Iijima, T.; Jimbo, H.; Taguchi, M. J . Mol. Struct. 1986, 144, 381. (31) Beagley, B.; Hewitt, T. G. Trum. Furuduy SOC.1968, 64, 2561. (32) Hamada, Y.; Tsuboi, M.; Yamanouchi, K.; Kuchitsu, K. J . Mol. Struct. 1986, 146, 253. (33) Bock, H.; Goebel, I.; Havlas, Z.; Liedle, S.; Oberhammer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 187. (34) Takeuchi, H.; Konaka, S.; Kimura, M. J . Mol. Srruct. 1986, 146, 361.
4396
J. Phys. Chem. 1992,96.4396-4404
~ r o p y l a m i n e(120.1 ~ ~ (10)'). Similarly, the CNC angles of trimethylamine3' and TEA (1 10.6 (6)' and 111.5-1 14.7 (26)') are smaller than that of trii~opropylamine~~ (1 19.2 (3)'). Thus, structural deformation due to the steric effect of the isopropyl groups is found in the r(N-C) and LCNC of isopropyl compounds except for the r(N-C) of triisopropylamine. The r - re(4-21G) values of r(N-C), r(C
