CONFORMATIONAL AND STRUCTURAL
STUDIES OF ISOPROPYLAMINE FROM
TEMPERATURE DEPENDENT RAMAN
SPECTRA OF XENON SOLUTIONS AND AB
INITIO CALCULATIONS
JOSHUA J. KLAASSEN, IKHLAS D. DARKHALIL, JAMES R. DURIG
Isopropylamine, (CH2)2CHNH2
• Two stable conformers
• Microwave work shows
trans conformer only
• Trans form is predicted to
be most stable conformer
• energy predictions vary
significantly with the
addition of diffuse
functions
(A) observed Raman spectrum in xenon
(B) simulated Raman spectrum of the mixture
at -100°C with ΔH = 113 cm-1
(C) simulated spectrum of pure trans
(D) simulated spectrum of pure gauche
Raman spectra of the xenon solution (Top)
Infrared spectra of the vapor phase (Bottom)
(A) 1510 – 1310 cm-1
(B) 1300 – 1100 cm-1
(C)
900 – 700 cm-1
(A) Infrared spectra of
the vapor phase
(B) Raman spectra of
the xenon solution
Table 1. Observed and calculateda frequencies (cm -1) for trans isopropylamine.
Vib.
No.
A
A
Approximate Descriptions
ab
initio
fixed
scaledb
13
C-N stretch
14
NH2 wag
918
871
15
C-C-C symmetric stretch
854
16
C-C-N deformation, in-plane
489
28
C-C-C antisymmetric stretch
29
CH3 rock, in plane, out-of- phase
30
CH3 rock, in plane, out-of- phase
1042
989
IR
int.
6.5
Raman
act.
7.6
IR
Gas
978
Raman
Xe soln.
976
IR
Solid
P.E.D. c
989
33S13,23S12,14S14,14S11,10S15
4
96
58S14,15S6,10S15,10S12
34
66
57S15,25S13,10S11
69
31
62S16,15S17
68
32
130.1
4.2
786
793
810
810
1.8
8.1
818
818
814/816
474
13.0
0.8
472
472
471
1081
1026
0.8
3.9
1030
1025
1039/1055
48S28,37S27,10S29
-
-
1001
949
0.5
2.9
951
948
948
69S29,23S28
-
-
907
82S30,11S25
-
-
-
-
963
912
0.5
0.3
918
918
85S31
31 C-C-N deformation, out-of- plane
417
413
5.2
0.3
404
405
397
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/amu), and potential energy distributions (P.E.D.s)
b
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH stretches and deformations, 0.70 for the NH2 torsion, 1.0 for heavy atom bends, and 0.90 for all other modes.
c
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
a
Band
Contours
B*
C*
Table 2. Observed and calculateda frequencies (cm -1) for gauche isopropylamine.
Vib.
Approximate Descriptions
No.b
ab
initio
fixed
scaledc
IR
int.
Raman
act.
IR
gas
Raman
Xe soln.
P.E.D.
e
13
C-N stretch
1083
1026
2.8
3.7
1030
1025
27
NH2 twist
1012
960
0.7
6.5
951
948
35S27,19S11,13S13,12S28
29
CH3 rock, in plane, out-of- phase
990
939
14.8
4.6
940
938
30
CH3 rock, in plane, out-of- phase
957
907
2.9
0.7
918
15
NH2 wag
946
897
143.9
1.9
14
C-C-C symmetric stretch
846
802
7.6
16
C-C-N deformation, in-plane
478
462
1.2
30S13,43S12,14S14
Band
Contours
*
A
B*
C*
18
2
80
-
78
22
45S29,34S28
44
54
2
918
79S30,10S25
7
91
2
826
830
55S15,17S29,14S6
46
54
-
9.5
810
811
62S14,24S13,10S11
26
74
-
0.4
459
463
58S16,18S17
80
17
3
80S31
97
3
31 C-C-N deformation, out-of- plane
423
417
13.1
1.0
406
405
4
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å /amu), depolarization ratios and potential energy
distributions (P.E.D.s)
b
In order of decending frequency
c
MP2(full)/6-31G(d) fixed scaled frequencies with factors of 0.88 for CH stretches and deformations, 0.70 for the NH 2 torsion,1.0 for heavy atom bends, and 0.90 for
all other modes.
e
Calculated with MP2(full)/6-31G(d) and contributions of less than 10% are omitted.
*
A, B and C values in the last three columns are percentage infrared band contours.
a
Variable temperature xenon
Table 3.
Temperature and intensity ratios of the trans and gauche bands of isopropylamine.
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
1/T (10-3 K-1)
I472 / I463
I818 / I463
I818 / I3380
I818/ I3314
I3390 / I463
I3325/ I463
I472/ I3380
I472/ I3314
4.584
4.692
4.923
5.177
4.460
5.775
0.781
0.792
0.802
0.844
0.875
0.906
1.047
1.089
1.151
1.193
1.250
1.302
2.233
2.322
2.456
2.544
2.667
2.778
1.879
1.953
2.065
2.140
2.243
2.336
0.448
0.474
0.495
0.521
0.547
0.578
0.990
1.042
1.068
1.109
1.141
1.193
1.667
1.689
1.711
1.800
1.867
1.933
1.402
1.421
1.439
1.514
1.570
1.626
85  7
118  6
118  6
118  6
134  5
95  6
84  7
84  7
1/T (10-3 K-1) I3390 / I3380
I3390 / I3314
I3325 / I3380
I3325 / I3314
I3390 / I830b
I3325 / I830b
I818 / I830b
I472 / I830b
0.956
1.011
1.056
1.111
1.167
1.233
0.804
0.850
0.888
0.935
0.981
1.037
2.111
2.222
2.278
2.367
2.433
2.544
1.776
1.869
1.916
1.991
2.047
2.140
0.729
0.771
0.805
0.847
0.890
0.941
1.610
1.695
1.737
1.805
1.856
1.941
1.703
1.771
1.873
1.941
2.034
2.119
1.271
1.288
1.305
1.373
1.424
1.475
134  5
134  5
95  6
95  6
135  4
95  6
118  6
85  7

Ha
T(C)
Liquid
xenon
50.0
60.0
70.0
80.0
90.0
100.0
4.584
4.692
4.923
5.177
4.460
5.775

Ha
T(C)
Liquid
xenon

Ha
a
b
50.0
60.0
70.0
80.0
90.0
100.0
1/T (10-3 K-1) I793b/ I3380
4.584
4.692
4.923
5.177
4.460
5.775
I793b/ I3314
I793/ I830b
I793b / I463
0.440
0.442
0.444
0.467
0.489
0.511
0.336
0.355
0.374
0.393
0.411
0.430
0.305
0.322
0.339
0.356
0.373
0.390
0.188
0.198
0.208
0.219
0.229
0.240
131  7
131  8
131  7
130  6
Average value H = 113  2 cm-1 (1.35  0.03 kJ mol-1) with the trans conformer the more stable form and the statistical
uncertainty (1σ) obtained by utilizing all of the data as a single set.
The 830 and 793 are NH2 wag.
Table 4. Calculated energies (hartree) and energy differences (cm-1) for
the two conformers of isopropylamine
Basis Set
6-31G(d)
6-31+G(d)
6-311G(d,p)
6-311+G(d,p)
6-311G(2d,2p)
6-311+G(2d,2p)
6-311G(2df,2pd)
6-311+G(2df,2pd)
Average
a
MP2(full)
a
b
trans
gauche
0.863972
72
0.877327
228
1.062299
44
1.069475
204
1.116801
85
1.122603
202
1.187136
84
1.192526
197
71 / 208c
B3LYP
b
trans
gauche
1.487268
-4
1.497602
117
1.544412
15
1.549361
125
1.552714
16
1.557380
134
1.556893
26
1.561466
138
13 / 129c
a
Energy of conformer is given as –(E+173) H.
Difference is relative to trans form and given in cm-1.
c
Average without diffuse functions / Average with diffuse functions.
b
Table 5. Structural parameters (Å and degree) and rotational constants (MHz) for trans
isopropylamine.
Parameter
a
b
c
MP2(full)/
6-311+G(d,p)
B3LYP/
6-311+G(d,p)
EDa
MWb
adjusted
r0d
r N1-C2
1.466
1.473
1.469(13)
1.49(2)
1.465(3)
r C4-C2
1.522
1.529
1.529(5)
1.527*
1.530(3)
r C5-C2
1.522
1.529
1.529(5)
1.527*
1.530(3)
r N1-H6
1.016
1.015
1.031*
1.019(3)
r N1-H7
1.016
1.015
1.031*
1.019(3)
 N1C2C4
108.6
108.9
108.9(9)
108(2)
108.9(5)
 N1C2C5
108.6
108.9
108.9(9)
108(2)
108.9(5)
 C4C2C5
111.4
111.7
114.4(16)
111.8*
111.0(5)
 H6N1H7
106.9
107.2
106.0
 C2N1H6
110.3
110.9
111.5*
109(2)
110.3(5)
 C2N1H7
110.3
110.9
111.5*
109(2)
110.3(5)
τH6N1C2H3
60.0
60.0
59.0(5)
τH7N1C2H3
-60.0
-60.0
-59.0(5)
A
8391.30
8308.59
8331.92(2)
8332.77
B
8016.30
7934.61
7977.32(2)
7977.08
C
4689.37
4629.71
4657.17(8)
4656.62
106.9(5)
T. Iijima, T. Kondou and T. Takenaka, J. Mol. Struct., 445 (1998) 23.; asterisk indicates fixed values.
Structural parameters and dipole moments from S. C. Mehrotra, L. L. Griffin, C. O. Britt and J. E. Boggs,
J. Mol. Spectrosc., 64 (1977) 244., with rotational constants from Ch. Keussen and H. Dreizler, Z.
Naturforsch., 46a (1991) 527.; asterisk indicates assumed values.
Adjusted structural parameters with experimental rotational constants taken from Ch. Keussen and H.
Dreizler, Z. Naturforsch., 46a (1991) 527.
Table 6.
Comparison of rotational constants obtained from ab initio MP2(full)/6311+G(d,p) predictions, experimental valuesa from microwave spectra,
and adjusted r0 structural parameters for trans isopropylamine.
Conformer
(CH3)2CHNH2
(CH3)2CHND2
a
Rotational
MP2(full)/
Experimentala Adjusted r0
constants 6-311+G(d,p)
||
A
8391.30
8331.92(2)
8332.77
0.85
B
8016.30
7977.32(2)
7977.08
0.24
C
4689.37
4657.17(8)
4656.62
0.55
A
7843.64
7806.17(1)
7806.42
0.25
B
7537.32
7490.59(2)
7489.77
0.82
C
4360.70
4331.79(8)
4332.33
0.54
Ch. Keussen and H. Dreizler, Z. Naturforsch., 46a (1991) 527.
Conclusions
The utilization of liquid rare gases as solvents for enthalpy determinations has
many advantages as well as disadvantages.
The advantages are:
1. Bands are very narrow
2. Very accurate measurement of the temperature
3. Fairly large temperature range
4. Little interaction of solvent with solute molecules
5. Small enthalpy changes can be measured
6. Band areas easily measured
7. Long paths for the liquids so very dilute solutions can be used
8. Solvent has no absorption bands
9. Usually several conformer pairs can be measured
10. Very low statistical uncertainty of determined values
Conclusions
The disadvantages are:
1. Limited solubility of many polar molecules
2. Difficult to have very dry xenon so water can interfere
3. At low temperatures sample may deposit on the window
Acknowledgement
J. R. D. acknowledges the University of Missouri-Kansas City for a Faculty Research
Grant for partial financial support of this research.
Thank you!!!