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THEO CHEM
Journal of Molecular Structure (Theochem) 309 (1994) 267-277
Ab initio studies of molecules with N-C-C-O units. Part 1. 2-Aminoethanal
Luis Carballeira*, Ignacio PCrez-Juste
Departamento de Quimica Pura y Aplicada, Lab. Quimica Fisica. Facultad de Ciencias, Apdo. 874, Vigo, Spain
(Received 16 November 1993; accepted 10 December 1993)
Abstract
A complete conformational analysis of 2-aminoethanal (2AE) has been carried out using the 6-31G** basis set. The curve corresponding to the barrier of rotation of the N-C-C=0 torsion was obtained and compared with the MM392 and the previously reported 4-21G curves. Geometrical trends relating to intramolecular hydrogen bonding were found and quantitatively discussed. Full geometry optimization MP2/6-31G**//6-3 lG** was performed for the stable con- formers found along the N-C-C=0 curve with different arrangements of the NH2 group.
1. Introduction
The detailed conformational analysis of simple compounds containing the N-C-C=0 unit is of chemical interest for a number of reasons. This unit is present in proteins and its conformational characteristics are important since it has torsional analogies with amino acid residues in peptides [ 1,2], and it is a simple model that facilitates an analy- sis of the possible influence of intramolecular hydrogen bonding on the stabilization of some conformations in extended forms of peptides [2(c)]. Moreover, it is present in the absolute struc- ture of domoic acid, a marine neurotoxin recently found to be responsible for an episode of amnesic poisoning produced by mussels [3]. Apart from its neuroexcitative functions, this acid has an extremely strong activity as an insecticide [4]; there- fore, the conformational analysis of this natural
*Corresponding author.
acid and other related compounds can help in the development of a new type of insecticide.
Despite these and other reasons, there is no experimental information on the structure of simple compounds with the N-C-C=0 unit, and very few theoretical studies have been carried out. With regard to the simplest compound of the series with the N-C-C=0 unit, 2-aminoethanal (2AE), Peters and Peters [5] have published an MO ab initio study, without geometry optimi- zation, and Van den Enden et al. [6] have carried out a detailed analysis of ten conformations of 2AE and the potential energy curve of the N-C-C=0 torsion, using the MO ab initio 4-2 1 G basis set and Pulay’s force method [6]. These authors predict an interesting flat valley of potential energy for the torsion mentioned, between 150 and 210”, similar to that found in the methyl ester of glycine [7], which would support the flexibility of the 4 angles of the helical forms of peptide systems. They also conclude that the inversion of the relative stabilities
0166-1280/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03651-Z
268 L. Carballeira, I. PPrez-Juste/J. Mol. Struct. [Theochem) 309 (1994) 267-277
of comparable conformations of NH-CH*--COR in monomers and dipeptide systems of glycine could be responsible for the correlations found between the monomer, dimer and polymer states of glycine residues in proteins. This inversion could be due to the specific intramolecular inter- actions of the monomer.
One of our objectives is the parameterization of a force field of molecular mechanics for the correct conformational treatment of the N-C-C=0 unit, according to the MM2 or MM3 methodology of Allinger et al. (see Ref. 8, Chapter 3), since, to date, none has been elaborated using information provided by model compounds. This field is necessary, for example, for the structural study of domoic acid and derivatives, since its size rules out the use of quantum methods. Because of the absence of this information, we intend to develop a “quantum mechanical force field” from ab initio data, which seems promising at least for the deter- mination of the constants of torsion potential (see Ref. 8, Chapter 4). Therefore, the specific objective of this study is the conformational analysis of 2AE in order to investigate the effect of changing the basis set on previous 4-21G results 161, at the
same time studying, in a quantitative manner, conformational aspects which have previously only been estimated. It should be noted that this involves differences in relative energy between stable conformers of 2AE, about 1 to 2 kcalmol-’ [6], and the presence of large flat regions in the curves or surfaces of potential energy. It is known that both facts can be sensitive to differ- ences between programs and basis sets, and to convergence criteria for the SCF cycle and geomet- rical optimization.
2. Method
Hehre et al. [9] consider that the 6-31G* basis set is a valid tool for obtaining conformational relative energies of various types of compounds, but when dealing with weak interactions of hydrogen bonding, it is convenient to add some extension in the phenomenon of polarization (see Ref. 8, Chapter 5). Therefore, for this and subse- quent investigations of the series, the use of the 6-31G** basis set has been selected, occasionally corrected with punctual MP2 calculations [IO].
N4
H7
H5
Fig. 1. Atom numbering and CG conformation of 2-aminoethanal.
L. Carbaileira, I. Pe’rez-Juste/J. Mol. Struct. (Theochem) 309 (1994) 267-277 269
N-C-C=0 Torsion
Fig. 2. (a) Representation of 6-31G** relative conformational energy against the values of the N-C-C=0 dihedral (Table 2). The curve was adjusted to a five-term Fourier series. (b) MM392 curve obtained driving the N-C-C=0 torsion.
All calculations were executed using the HONDO
8.3 program [l l] and the parameters of the incor- porated 6-3 1G basis set. Polarization for C, N, 0 (d exponent = 0.80) and H (p exponent = 1.10) was introduced. The geometries of the located confor- mers were obtained without any constraints until the convergence threshold on the maximum component of the gradient was less than 0.0005. The geometries of conformations which are not energy minima were obtained by constraining an internal coordinate (torsion), and adopting the same convergence criterion.
The notation describing the conformation is composed of two letters. The first specifies the N4C3C201 torsion (Fig. 1) as either cis (C) or trans (T); for the conformation obtained by constraining the dihedral angle, its value is speci- fied, followed by a subscript r. The second letter refers to the value of the H9N4C3C2 angle: 0” (C), 60” (G), 120” (E), 180” (T), -120” (E’), -60” (G’).
3. Results and discussion
3.1. Rotation barrier of the N-C-C=0 torsion
In order to determine the influence of the N-C-C=0 torsion on the stability of the mol- ecule, refined geometries of 2AE were obtained, fixing the value of the dihedral between 0 and
180” (see Table 1). In each of the geometries obtained, the NH2 group is in the G arrange- ment. Points were analyzed at different intervals in order to obtain a better fit of the curve corre- sponding to the rotation barrier.
From the energy values (Table 2), it can be observed that the energy minimum corresponds to an N-C-C=0 torsion equal to O”, the maxi- mum is reached at 74.94” with a height of 3.99 kcal mol-’ , and an energy plateau with higher energy than 0” (2.31 kcalmol-‘) is found between 150 and 180” (Fig. 2). As expected, changing the basis and polarization produce a decrease in absolute energy with respect to the 4-21G study [6]. The variation of barrier height is negligible, but the increase in relative energy of the plateau from 1.92-1.71 (4-21G) to 2.35-2.31 kcalmol-’ (6-31G**) for the region between 150 and 180” should be noted.
A series of coordinates presents periodical varia- tions along the rotation barrier (N4-C3, C3-C2 and H5-C2 bond lengths; C3-C2-01, N4-C3-C2 and H5-C2-01 angles). The values of these coordi- nates change from short to long and finally back to short values. These trends are due to bond delocalization effects for the conformations with N-C-C=0 at 0 and 180” (where there is some partial double bond character) and to their decou- pling in non-planar forms. General geometrical trends along the N-C-C=0 curve are similar to
Tab
le
1
6-31
G**
st
ruct
ural
pa
ram
eter
s op
timiz
ed
for
vari
ous
conf
orm
atio
ns
of 2
-am
inoe
than
al.
In e
ach
conf
orm
atio
n th
e N
-C-C
=0
angl
e w
as
fixe
d at
th
e va
lues
in
dica
ted
C*G
30
,G
60,G
70
,G
75,G
80
,G
90,G
Bon
d le
ngrh
(A
)
c2-0
1
C3-
C2
N4-
C3
H5-
C2
H6-
C3
H77
C3
H8-
N4
H9-
N4
Bon
d an
gle
(deg
)
C3-
C2-
01
N4-
C3-
C2
H5-
C2-
01
H5-
C2-
C3
H6-
C3-
C2
H66
C3-
N4
H7-
C3-
C2
H7-
C3-
N4
H7-
C3-
H6
H8-
N4-
C3
H9-
N4-
C3
H9-
N4-
H8
Tor
sion
al
angl
e (d
eg)
N4-
C3-
C2-
01
N4-
C3-
C2-
H5
H6-
C3-
C2-
01
H6-
C3-
C2-
H5
H7-
C3-
C2-
01
H7-
C3-
C2-
H5
H88
N44
C3-
C2
H8-
N4-
C3-
H6
H8-
N4-
C3-
H7
H9-
N4-
C3-
C2
H9-
N4-
C3-
H6
H99
N44
C3-
H7
1.18
82
1.18
88
1.18
91
1.18
92
1.18
90
1.18
90
1.18
89
1.51
21
1.51
65
1.52
16
1.52
12
1.51
98
1.51
90
1.51
62
1.43
90
1.44
25
1.44
94
1.45
10
1.45
26
1.45
29
1.45
25
1.09
82
1.09
80
1.09
70
1.09
77
1.09
79
1.09
82
1.09
88
1.08
83
1.08
59
1.08
52
1.08
49
1.08
47
1.08
48
1.08
52
1.08
83
1.08
90
1.08
59
1.08
41
1.08
41
1.08
36
1.08
34
1.00
05
1.00
10
1.00
05
1.00
04
1.00
05
1.00
04
0.99
96
1.00
05
1.00
03
1.00
02
1.00
01
1.00
03
1.00
02
1.00
04
124.
16
123.
17
122.
13
122.
30
122.
52
122.
77
123.
17
115.
98
115.
46
114.
05
113.
48
112.
85
112.
75
112.
51
120.
80
120.
68
120.
40
120.
26
120.
27
120.
26
120.
32
115.
04
116.
05
117.
47
117.
43
117.
16
116.
91
116.
42
107.
49
109.
10
109.
91
110.
09
110.
23
110.
17
110.
07
109.
94
109.
52
108.
95
108.
89
108.
90
108.
79
108.
73
107.
49
106.
38
107.
27
107.
54
107.
91
107.
96
107.
98
109.
94
109.
90
108.
89
108.
69
108.
68
108.
69
108.
97
105.
45
106.
05
107.
56
108.
00
108.
15
108.
38
108.
49
110.
40
110.
14
110.
18
110.
58
110.
43
110.
66
111.
01
110.
40
110.
60
111.
36
111.
70
111.
75
111.
97
112.
34
106.
41
107.
00
107.
78
108.
11
108.
02
108.
26
108.
49
.oo
30.0
0 60
.00
70.0
0 74
.94
80.0
0 90
.00
180.
00
-153
.77
-120
.83
-108
.87
-102
.55
-97.
18
-86.
65
123.
46
153.
81
-177
.33
-167
.67
-163
.06
-158
.25
-148
.57
-56.
54
-29.
96
1.85
13
.45
19.4
4 24
.57
34.7
8
-123
.46
-92.
20
-60.
66
-50.
24
-45.
16
-40.
08
-30.
30
56.5
4 84
.03
118.
51
130.
88
137.
34
142.
74
153.
05
-58.
69
-50.
21
-55.
14
-56.
09
-56.
38
-56.
61
-58.
27
179.
14
-173
.80
-178
.34
-179
.08
-179
.12
-179
.14
179.
54
63.4
8 70
.09
64.6
2 63
.50
63.2
8 63
.04
61.4
6
58.6
9 67
.85
64.4
1 64
.36
63.8
8 64
.27
63.4
1
-63.
48
-55.
73
-58.
79
-58.
63
-58.
86
-58.
26
-58.
78
-179
.14
-171
.85
-175
.84
-176
.05
- 17
6.46
-
176.
08
-176
.87
Bon
d le
ngt
h (A
) c2
-01
C3-C
2
N4-C
3
HS
-C2
H6-C
3
H7-C
3
H8-N
4
H9-N
4
Bon
d a
ngle
(d
eg)
C3-C
2-0
1
N4-C
3-C
2
H5-C
2-0
1
HS
-C2-C
3
H6-C
3-C
2
H6-C
3-N
4
H7-C
3-C
2
H7-C
3-N
4
H7-C
3-H
6
H8-N
4-C
3
H9-N
4-C
3
H9-N
4-H
8
Tor
sion
al a
ngle
(a
kg)
N4-C
3-C
2-0
1
N4-C
3-C
2-H
5
H6-C
3-C
2-0
1
H6-C
3-C
2-H
5
H7-C
3-C
2-0
1
H7-C
3-C
2-H
5
H8-N
4-C
3-C
2
H8-N
4-C
3-H
6
H8-N
4-C
3-H
7
H9-N
4-C
3-C
2
H9-N
4-C
3-H
6
H9-N
4-C
3-H
7
1.1
885
1.1
882
1.1
877
1.1
875
1.1
871
1.1
869
1.1
869
1.5
148
1.5
142
1.5
139
1.5
150
1.5
168
1.5
189
1.5
187
1.4
529
1.4
514
1.4
502
1.4
478
1.4
445
1.4
433
1.4
428
1.0
993
1.0
994
1.1
001
1.1
000
1.1
006
1.1
001
1.1
009
1.0
857
1.0
859
1.0
864
1.0
878
1.0
876
1.0
868
1.0
854
1.0
828
1.0
825
1.0
827
1.0
830
1.0
836
1.0
841
1.0
854
1.0
001
1 .o
oOO
0.9
996
0.9
995
0.9
996
0.9
992
0.9
993
1.0
001
1 .O
Ooo
0.9
997
1.0
000
0.9
991
0.9
991
0.9
993
123.7
0
124.1
0
124.4
1
124.4
8
123.9
4
123.5
1
123.4
1
112.2
4
112.6
3
113.2
9
114.3
1
115.4
6
116.0
5
116.2
1
120.3
3
120.3
7
120.5
3
120.6
6
120.8
6
120.8
4
120.9
2
115.8
6
115.4
4
115.0
3
114.8
6
115.2
0
115.6
4
115.6
6
109.9
3
109.5
4
108.8
5
107.9
3
107.3
1
107.0
1
107.4
1
108.8
1
108.7
4
108.6
2
108.6
0
108.8
1
109.0
9
109.5
7
108.1
5
108.1
4
108.2
3
108.2
2
107.8
8
107.9
0
107.4
1
109.1
3
109.3
7
109.6
9
110.0
5
110.2
6
110.0
4
109.5
7
108.5
1
108.3
2
108.0
2
107.4
9
106.7
4
106.2
8
106.1
8
111.3
0
111.6
4
111.9
0
112.0
5
112.4
6
112.5
4
112.6
7
112.4
1
112.5
2
112.6
0
112.5
2
112.6
9
112.7
3
112.6
7
108.5
1
108.6
6
108.6
3
108.5
7
108.7
6
108.7
4
108.6
8
100.0
0
110.0
0
120.0
0
135.0
0
150.0
0
165.0
0
180.0
0
-76.2
2
-66.7
5
-57.8
5
-44.3
1
-30.5
9
-16.0
7
0.0
0
-138.7
5
-128.8
4
-119.0
4
-104.0
6
-88.5
0
-72.9
8
-56.9
3
45.0
3
54.4
0
63.1
1
76.6
3
90.9
1
105.9
5
123.0
7
-20.4
4
-10.9
9
-1.8
7
12.0
0
26.1
8
41.0
4
56.9
3
163.3
4
172.2
6
-179.7
2
-167.3
1
-154.4
1
- 140.0
4
-123.0
7
-58.6
7
-58.9
4
-59.2
1
-59.6
8
-59.7
2
-61.6
5
-61.7
0
179.4
4
179.4
5
179.7
1
179.7
5
179.5
9
177.4
4
176.3
6
61.2
1
61.3
4
61.8
4
62.3
3
62.8
5
61.2
1
60.2
3
63.3
0
63.5
5
63.4
9
63.0
0
63.6
5
61.7
9
61.7
0
-58.5
0
-58.0
6
-57.5
9
-57.5
7
-57.0
4
-59.1
2
-60.2
3
-176.8
3
-176.1
7
-175.4
6
-174.9
9
-173.7
8
-175.3
6
-176.3
6
100,G
llO
rG
120,G
135,G
150,G
165,G
T,G
212 L. Carballeira, I. Pkrez-Juste/J. Mol. Struct. (Theochem) 309 (1994) 267-277
Table 2
HF/6-31G**//6_31G** total energies (E, hartree; El, kcalmol-‘)
and relative energies (E,, kcalmol-‘) for the conformations
defined in Table 1
E El E,
C,G -207.948709 - 130489.80 0.00
30,G -207.946520 - 130488.43 1.37
60,G -207.942888 -130486.15 3.65
70,G -207.942397 -130485.84 3.96
75,G -207.942352 -130485.81 3.99
80,G -207.942404 -130485.84 3.96
90,G -207.942750 - 130486.06 3.74
100,G -207.943270 - 130486.39 3.41
110,G -207.943786 -130486.71 3.09
120,G -207.944233 -130486.99 2.81
135,G -207.944716 - 130487.29 2.51
150,G -207.944961 - 130487.45 2.35
165,G -207.945026 -130487.49 2.31
TsG -207.945027 -130487.49 2.31
those deduced with 4-21G [6]. Nevertheless, bond lengths are smaller due to polarization, with the exception of the C-H bonds which increase slightly.
The curve obtained with MM392 [12] using the same procedure as for the ab initio data, that is driving the N-C-C=0 dihedral, can also be observed in Fig. 2. MM3 incorporates trial para- meters for the above-mentioned torsion, and, as expected, the discrepancy between the methods is very large. Although the initial part of the curve agrees qualitatively, remarkable differences are observed, a minimum appearing at 110” and an absolute maximum at 180”, with the opposite physical meaning to the 6-3 lG** results. This indi- cates the necessity of continuing with the ab initio studies or waiting for more secure experimental data to establish reliable MM3 parameters for the N-C-C=0 torsion.
3.2. Hydrogen bonding
Some authors [13] have deduced geometrical trends relating to intramolecular hydrogen bonding in molecules similar to 2AE. Briefly stated, when an N-H or O-H bond interacts with C=O, both tend to be elongated compared to non-interacting bonds and the C-X-H angle (where X = N, 0) tends to contract in order to optimize the interaction distance between the hydrogen and oxygen.
These tendencies can be found in some of the forms studied along the N-C-C=0 torsion curve. Thus, when N-C-C=0 is O”, the H&-N4 and H9-N4 bonds are equidistant from the oxygen of the carbonyl, and symmetrical hydrogen bonding can occur between them, with a five-membered ring closing each hydrogen atom. When the dihedral N-C-C=0 angle is 180”, however, no hydrogen bonding can occur. This assumption is confirmed by the fact that the H&-N4 and H9-N4 bonds are longer (1 .OOOS A) in the C,G conformation than in T,G (0.9993 A). The same occurs with the H8-N4-C3 and H9-N4-C3 angles which are smaller (110.40”) in C,G than in T,G (112.67”), in accordance with the trends already mentioned.
The existence of hydrogen bonding can also be observed in the variation of the C2=01 bond length along the curve, which has a larger value in C,G than T,G as a result of the previously men- tioned symmetrical hydrogen bonding. Moreover, this bond increases in the O-70” interval of the N-C-C=0 torsion due to the persistence of the hydrogen bonding with H8-N4, also confirmed by the fact that the H&N4 bond length stays at a constant value. The H9-N4 bond, however, which does not undergo hydrogen bonding, but takes on decreasing values. The existence of hydrogen bonding is also confirmed by analyzing the values of the H8-N4-C3 angle, which remains approximately constant, and that of H9-N4-C3, which increases over the whole interval.
A comparative analysis draws attention to trends which indicate that hydrogen bonding is less intense than that deduced with 4-21G [6]. Therefore, increments in the N-H bond lengths are 0.0047A (4-21G) and 0.0012A (6-31G**). In the same way, the variation in the H-N-C angle oscillates between 3.94” (4-21G) and 2.27” (6-31G**).
3.3. Variation of the position of the NH2 group
As previously mentioned, all the calculations were carried out with the G arrangement of the NH2 group, so that the H9N4C3C2 torsion remains at values of about 60”. Two more positions of this group, which also have N-H bonds staggered with respect to the remaining adjacent bonds, are favourable, and therefore,
Bon
d le
ngth
(A
) c2
-01
C3-
C2
N4-
C3
H5-
C2
H6-
C3
H7-
C3
HS-
N4
H9-
N4
Bon
d an
gle
(deg
j
C3-
C2-
01
N4-
C3-
C2
H5-
C2-
01
H5-
C2-
C3
H6-
C3-
C2
H6-
C3-
N4
H7-
C3-
C2
H7-
C3-
N4
H7-
C3-
H6
HS-
N4-
C3
H9-
N4-
C3
H9-
N4-
H8
Tor
sion
al a
ngle
(de
g)
N4-
C3-
C2-
01
N4-
C3-
C2-
H5
H6-
C3-
C2-
01
H6-
C3-
C2-
H5
H7-
C3-
C2-
01
H7-
C3-
C2-
H5
HS-
N4-
C3-
C2
HS-
N4-
C3-
H6
HS-
N4-
C3-
H7
H9-
N4-
C3-
C2
H9-
N4L
C3-
H6
H9-
N4-
C3-
H7
1.18
82
1.18
83
1.18
72
1.18
37
1.18
69
1.18
57
1.18
72
1.18
79
1.18
71
1.18
77
1.18
70
1.51
21
1.50
77
1.50
65
1.51
54
1.51
87
1.51
35
1.51
54
1.51
76
1.51
68
1.51
23
1.51
30
1.43
90
1.44
23
1.43
87
1.43
95
1.44
28
1.45
10
1.44
78
1.45
01
1.44
45
1.44
83
1.45
05
1.09
82
1.09
59
1.09
67
1.10
06
1.10
09
1.10
02
1.09
54
1.09
56
1.10
06
1.09
59
1.09
69
1.08
83
1.09
05
1.08
82
1.09
03
1.08
54
1.08
49
1.08
55
1.08
62
1.08
76
1.08
81
1.09
32
1.08
83
1.09
18
1.09
67
1.09
01
1.08
54
1.08
98
1.09
01
1.08
66
1.08
36
1.08
81
1.08
28
1.00
05
0.99
74
0.99
82
0.99
56
0.99
93
0.99
70
0.99
85
0.99
69
0.99
96
0.99
90
1.00
05
1.00
05
1 .oo
OO
0.
9986
0.
9957
0.
9993
0.
9991
1 .
oooo
0.
9972
0.
9991
1.
0003
0.
9989
124.
16
124.
21
124.
28
126.
30
123.
41
123.
52
123.
13
123.
02
123.
94
123.
54
123.
55
115.
98
112.
82
111.
54
114.
38
116.
21
113.
35
110.
80
111.
81
115.
46
110.
42
110.
43
120.
80
120.
84
120.
88
120.
64
120.
92
120.
72
121.
73
122.
04
120.
86
121.
69
121.
09
115.
04
114.
95
114.
81
113.
06
115.
66
115.
75
115.
13
114.
95
115.
20
114.
76
115.
36
107.
49
106.
87
107.
72
106.
08
107.
41
107.
34
106.
67
107.
08
107.
31
106.
39
107.
29
109.
94
111.
53
110.
33
112.
19
109.
57
111.
36
109.
13
112.
13
108.
81
108.
42
114.
25
107.
49
106.
13
105.
64
106.
26
107.
41
105.
84
107.
83
107.
10
107.
88
108.
19
107.
62
109.
94
113.
38
115.
31
112.
29
109.
57
111.
82
115.
27
112.
29
110.
26
115.
74
109.
94
105.
45
105.
58
105.
86
104.
95
106.
18
106.
72
106.
74
106.
05
106.
74
107.
25
107.
03
110.
40
112.
18
111.
80
113.
33
112.
67
112.
62
111.
56
112.
63
112.
46
111.
44
111.
60
110.
40
110.
35
111.
13
113.
31
112.
67
113.
17
111.
66
112.
69
112.
69
111.
48
111.
24
106.
41
108.
82
109.
24
109.
61
108.
68
108.
22
107.
54
108.
34
108.
76
107.
51
107.
30
0.00
6.
39
11.7
6 -1
.47
180.
00
-159
.33
175.
18
-178
.76
150.
00
152.
25
154.
82
180.
00
-173
.82
-170
.12
178.
75
0.00
22
.19
-5.7
2 1.
31
-30.
59
-28.
74
-25.
92
123.
46
129.
32
132.
98
122.
74
-56.
93
-35.
93
-66.
15
-55.
57
-88.
50
-90.
30
-80.
07
-56.
54
-50.
89
-48.
91
-57.
04
123.
07
145.
60
112.
95
124.
50
90.9
1 88
.71
99.1
9 -1
23.4
6 -1
18.3
5 -1
14.2
3 -1
25.9
4 56
.93
77.7
6 48
.18
57.8
3 26
.18
24.6
6 34
.81
56.5
4 61
.44
63.8
8 54
.29
-123
.07
-100
.71
-132
.71
-122
.10
- 15
4.41
-1
56.3
4 -1
45.9
2 -5
8.69
-1
30.3
7 -1
66.3
0 11
7.38
-6
1.70
-1
28.9
3 16
4.24
11
9.54
-5
9.72
16
6.29
72
.85
179.
14
109.
34
74.0
2 -3
.50
176.
36
109.
90
47.0
8 -0
.75
179.
59
50.1
0 -4
8.20
63.4
8 -9
.68
-45.
82
-121
.42
60.2
3 -9
.39
-72.
98
-120
.04
62.8
5 -7
0.40
-1
68.5
5 58
.69
-8.8
4 -4
3.94
-1
16.9
1 61
.70
-5.7
9 -7
5.38
-1
17.5
2 63
.65
-73.
59
-167
.35
-63.
48
-129
.14
-163
.63
122.
21
-60.
23
- 12
6.96
16
7.45
12
2.20
-5
7.04
17
0.21
71
.60
-179
.14
111.
84
76.5
3 4.
29
-176
.36
113.
75
47.4
0 2.
91
-173
.78
49.7
2 -4
8.75
Tab
le
3
6-31
G*
optim
ized
ge
omet
ries
fo
r st
able
co
nfor
mer
s an
d tr
ansi
tion
stat
es
defi
ned
in t
he
text
CC
cc
C
C’
CE
’ T
G
TC
T
G’
TE
’ 15
0G
150G
’ 15
0T
214 L. Carballeira, I. Ptkez-Juste/J. Mol. Struct. (Theochem) 309 (1994) 267-277
these possibilities were studied for the stable conformers found with N-C-C=0 equal to 0 and 180”. In both cases, because of the symmetry of the molecule, the favourable conformations are reduced to two, G and G’. The third favourable arrangement with the H9N4C3C2 angle equal to 180” (T) is equivalent to G’. Apart from these totally staggered favourable configurations, there are two totally eclipsed intermediate arrange- ments, C and E’, and another, E, with N4C3C201 equal to 120”, equivalent to C. After studying all the combinations of the NH2 group arrangements with the N-C-C=0 torsion values indicated, it was found that all the conformations with the staggered arrangement of the NH2 group were stable. Between these stable conformers, completely eclipsed transi- tion states were located. Full geometry optimization was carried out in all cases (Table 3).
energy of CE’ is due to the repulsion of the elec- tron lone pair of N and the C2=01 bond.
The hydrogen bonds formed by H9 and H8 with 01 in CG exist only for H9 in the transition state CC and the stable conformer CG’. In CG, H9-N4 and H8-N4 bond lengths are equal and longer than in any other case, for example, in CE’, where hydrogen bonding is not possible. Never- theless, for CC and CG’, the H9-N4 bond length is always greater than H8-N4 because of the existence of hydrogen bonding. Finally, in CE’ both lengths are practically equal and shorter than in any other case. The same trends can be observed in the C2=01 bond, which takes on the smaller value in the CE’ transition and the H9-N4-C3 and H8-N4-C3 angles, which always take smaller values than in CE’.
From the energy results (Table 4) it can be deduced that the most stable conformer is CG (N4C3C201 = 0” and H9N4C3C2 = 58.69”). For conformers with N-C-C=0 in cis, CG’ has a higher relative energy of 1.72 kcalmol-i, and there are two transition states between them: (CG-CC-CG’) 1.87 kcal mol-’ high and (CG’-CE’-CT) 5.67 kcal mol-’ high. The exist- ence of the transition states could be due to repulsion between eclipsed pairs. Therefore, in CC, despite the fact that H9 and 01 form hydrogen bonds (see below), H8-N4 and H7-C3 are strongly eclipsed. On the contrary, the high
For the conformers with N-C-C=0 in trans, the relative stabilities of the arrangements of the NH2 group are different. Thus, TG’ is the most stable and has an H9N4C3C2 angle equal to -75.38”, with a relative energy 0.59 kcalmol-’ lower than TG. The reason for the inversion of stability probably resides in the position of the H5-C2 bond with respect to the N-H bonds. Thus, the reason for TG’ being more stable is probably that the repulsions between the bonds mentioned are smaller, since H5-C2 is as far away as possible from both N-H bonds, and they are responsible for the high relative energy of TC, where H5-C2 and H9-N4 are strongly eclipsed.
Table 4
Total energies (E, hartree; El, kcal mol-‘) and relative energies (E,, kcal mol-‘) for the conformations defined in Table 3
HF/6-3lG**//6-3lG** MP2/6-3lG**//6-3lG**
E El Er E El E,
CG cc CG’ CE’
TG
TC
TG’
TE’
150G
150G’
150T
-207.948709 -130489.80 0.00 -208.586048 - 130889.74 .oo -207.945729 - 130487.93 1.87 -208.583306 -130888.02 1.72
-207.945970 - 130488.08 1.72 -208.583290 -130888.01 1.73
-207.939680 - 130484.14 5.67 -208.576576 -130883.79 5.94
-207.945027 - 130487.49 2.31 -208.581850 -130887.10 2.63
-207.939915 - 130484.28 5.52 -208.576701 -130883.87 5.87
-207.945973 - 130488.08 1.72 -208.582200 -130887.32 2.41
-207.944878 -130487.40 2.40 -208.580869 -130886.49 3.25
-207.944961 - 130487.45 2.35 -208.581843 -130887.10 2.63
-207.946487 -130488.41 1.39 -208.582755 -130887.67 2.06
-207.944642 - 130487.25 2.55 -208.580849 -130886.48 3.26
Tab
le
5
Som
e 6-
3 lG
**
stru
ctur
al
para
met
ers
and
ener
gies
(E
, ha
rtre
e;
.&I,
, kca
l m
ol-‘
) of
va
riou
s co
nfor
mat
ions
of
2-
amin
oeth
anal
in
w
hich
th
e N
4C3C
201
and
H9N
4C3C
2
tors
ions
w
ere
rest
rict
ed
to
the
valu
es
indi
cate
d
Cr6
0,
C,3
0,
C,O
, c,
-
30,
C,
- 60
, c,
-
90,
c,
- 12
0,
Bon
d le
ngth
(d
) c2
-01
C3-
C2
N4-
C3
H5-
C2
H6-
C3
H9-
N4
1.18
82
1.18
83
1.18
79
1.18
75
1.18
58
1.18
43
1.18
37
1.51
21
1.51
07
1 SO
82
1.50
54
1.50
84
1.51
28
1.51
53
1.43
90
1.44
21
1.44
35
1.44
02
1.43
95
1.44
08
1.43
94
1.09
82
1.09
80
1.09
65
1.09
62
1.09
77
1.10
01
1.10
07
1.08
83
1.09
07
1.09
12
1.08
99
1.08
90
1.08
86
1.09
08
1.00
05
1.00
11
1.00
06
0.99
87
0.99
97
0.99
87
0.99
59
Bon
d an
gle
(deg
) C
3-C
2-01
N4-
C3-
C2
H5-
C2-
C3
H6-
C3-
C2
H6-
C3-
N4
H?-
C3-
H6
H9-
N4-
C3
H9-
N4-
H8
124.
16
124.
14
124.
29
124.
49
125.
21
125.
87
126.
28
115.
98
115.
44
113.
65
112.
02
112.
43
113.
40
114.
32
115.
04
114.
97
114.
80
114.
64
113.
82
113.
49
113.
06
107.
49
106.
29
106.
12
107.
02
106.
79
105.
81
105.
86
109.
94
110.
54
111.
43
111.
10
109.
65
109.
64
111.
87
105.
45
105.
31
105.
48
105.
72
105.
83
105.
33
104.
95
110.
40
109.
93
109.
87
110.
89
111.
45
112.
14
113.
52
106.
41
106.
10
107.
78
109.
39
108.
08
107.
64
109.
45
Tor
sion
al
angl
e (d
eg)
N4-
C3-
C2-
0 1
N4-
C3-
C2-
H5
H6-
C3-
C2-
01
H6-
C3-
C2-
H5
H9-
N4-
C3-
C2
H9-
N4-
C3-
H6
H9-
N4-
C3-
H7
0.00
0.
00
0.00
-0
.00
0.00
0.
00
0.00
180.
00
-179
.04
- 17
9.02
17
9.66
-1
79.8
8 -1
79.8
9 17
9.81
123.
46
122.
94
122.
81
121.
99
120.
30
120.
21
123.
60
-56.
54
-56.
10
-56.
21
-57.
67
-59.
58
-59.
68
-56.
59
58.6
9 29
.95
0.00
-3
0.00
-6
0.00
-9
0.00
-1
16.9
1
-63.
48
-90.
71
-119
.84
-149
.64
-178
.63
151.
99
122.
79
-179
.14
153.
04
121.
80
90.6
6 62
.05
33.6
0 4.
85
E
-207
.948
709
-207
.947
598
-207
.945
795
-207
.945
430
-207
.944
605
-207
.941
695
-207
.939
671
4 0.
00
0.70
1.
83
2.06
2.
58
4.40
5.
67
276 L. Carballeira, I. Pirez-Juste/J. Mol. Struct. (Theochem) 309 (1994) 267-277
-________-------__
d -120 -90 -60 -30 0 30 60
H9N4C3C2 Torsion
Fig. 3. (a) 6-31G** plot of relative energy of H9N4C3C2 torsion maintaining N4C3C201 fixed at 0” (see text). The line through the points was fitted to a five-term polynomial. (b) MM392 plot.
This can be confirmed by analyzing the N4-C3-C2 angle. Its smaller values are in TG’ and TE’, when the H5-C2 bond is further away from the N-H bonds. Nevertheless, in the other two conformers, the angle has larger values due to the repulsions between N-H and H5-C2.
To analyze the influence of electron correlation to some extent, MP2 corrected energies were calcu- lated for the stable conformers and transition states (Table 4). In general, the relative energy increases, except for CC. On the contrary, the relative energies of the G forms decrease with respect to G’ in the region of the energy valley: TG-TG’ goes from 0.59 to 0.22 kcalmol-t and, as it will be shown, 150G- 150G’ goes from 0.96 to 0.57 kcal mol-' .
The reason for the remarkable fact that the energy of the CC transition is lower than that of the CC’ conformer probably resides in the compari- son of different planes of potential energy surface, since the N-C-C=0 dihedrals are clearly different: 6.39” (CC) and 11.76” (CG’), and in both cases far from the most stable conformer at 0” (CG). In order to confirm this, conformations were refined by constraining the value of N4C3C201 to 0” and the value of H9N4C3C2 to intervals of 30” between -120 and 60” (Table 5). As expected, the minimum energy was obtained at 60” and maximum at 120”, but between 0 and -50” a flat area can be observed (Fig. 3), indicating that for points situated on the same plane of the potential energy surface (that is,
points with the same N-C-C=0 torsion value) the CC- and CG’-type conformers will have very similar energies. The same curve was obtained with MM392, in this case agreeing acceptably with 6-31G** data, although MM3 parameters could be refined to obtain better quantitative agreement.
Finally, the arrangements of the NH2 group for the conformation with N-C-C=0 equal to 150” were investigated. The interest of this research lies in the existence of three different staggered forms, due to loss of symmetry with respect to conformations with N-C-C=0 at 0 or 180”. As expected, the relative energy of these conformers is similar to that found for those with N-C-C=0 equal to 180”, although a third form appears, 150T, which is the least stable. The reasons for the rela- tive stability of the conformers with N-C-C=0 equal to 150” are similar to those described for the T forms. In 150G, the most stable one, the H5-C2 bond is in a favourable position with respect to the N-H bonds and the electron lone pair of N, while in 150G and 150T the H5-C2 bond interacts with the H9-N4 and H8-N4 bonds respectively. More- over, in 150T the electron lone pair of N is on the same side of the molecule as the C=O bond. These results are different to those previously described [6]. Although the order of relative energies coin- cides, G’ < G < T, the difference of energies (0.00, 0.96, 1.16 kcal mol-i) differs from that estimated with 4-21G (approximately 0.0, 1.2,2.0 kcal mol-‘).
L. Carballeira. I. Pkrez-Juste/J. Mol. Struct. (Theochem) 309 (1994) 267-277 277
From the above it can be concluded that, although for conformations of 2AE with N-C-C=0 equal to 0” the most stable arrange- ment of the NH2 group is G, when analyzing the region of the energy plateau, the most stable forms will have the G’ arrangement.
Acknowledgements
The authors are indebted to the Xunta of Galicia and the University of Vigo for financial support granted for the realization of this work. I.P.J. also wishes to thank the Xunta of Galicia for the award of a Ph.D. Curses grant.
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