Journal of Molecular Structure (Theochem), 205 (1990) 235-244 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

235

AB INITIO STUDIES OF MOLECULES WITH N-C-O UNITS

Part I. Methylaminomethanol, I-methoxymethylamine and l-methoxy-N,N-dimethyl-methylamine

LUIS CARBALLEIRA, BERTA FERNANDEZ, RICARDO A MOSQUERA and MIGUEL A. RiOS

Departamento de Quimica Fisica, Uniuersidad de Santiago, Santiago de Compostela, E-15706, Galicia (Spain)

(Received 2 May 1989 )

ABSTRACT

The conformations of various conformers of the molecules methylaminomethanol, l-methox- ymethylamine and 1-methoxy-N,N-dimethylmethylamine, all of which contain an N-C-O unit, were determined by an ab initio method using the 4-21G basis set and complete geometrical op- timization. The results were interpreted in terms of anomeric interactions: the most stable con- formers of all three molecules have their lone pairs antiperiplanar to their polar bonds.

INTRODUCTION

Very little experimental research on the structures of molecules containing N-C-O units has been carried out, the main source of information being a ‘H- NMR study of l-methoxy-N,iV-dimethylmethylamine [ 1 ] (DMOMA) and a 13C-NMR study of methylaminomethanol [ 21. The only theoretical research on molecules of this class has concerned its simplest member, aminomethanol (AM) ; the geometries of four conformers of this molecule have been optimized using the 4-21G//4-21G basis set and the results [3] interpreted in terms of anomeric interactions (which have also been invoked in other theoretical stud- ies) [4-61; its heat of formation [ 71, conformer stabilities [8] and potential- energy surface [9] have likewise been studied by ab initio methods, and the energies of its conformers in vacua and in aqueous solution have also been studied [lo]. In this paper we report the results of applying Pulay’s method [ 111 and program [ 121 to the ab initio conformational analysis of the three N-C-O molecules methylaminomethanol (MMA), 1-methoxymethylamine (OMA) and DMOMA using the 4-21G basis set [ 131 and complete geometrical optimization. The results should prove useful for the parametrization of mo- lecular mechanics force fields that would allow theoretical research on large

0166-1280/90/$03.50 0 1990 Elsevier Science Publishers B.V.

236

N-C-O molecules, some of which are of biological and/or pharmaceutical in- terest [ 141.

Geometrical optimization was performed starting from all the staggered con- formations of the molecules studied (nine for MMA and five each for OMA and DMOMA), and was continued with no constraints until the Cartesian components of the forces on the atoms were all less than 0.001 mdyn. In what

H \ 10 H9 \ \ \

H 1 P

,“\H,,

3

DMOMA

Fig. 1. The atom numbering used for MMA, OMA and DMOMA.

237

follows, conformers are denoted by pairs of letters denoting the main dihedral angles (M for 60’) G for - 60’ and A for 180” ), the first letter corresponding to the angle X-O-C-N (X = H in MMA, X = C in OMA and DMOMA) and the second to the angle Y-N-C-O (Y = C4 in MMA, Y = C5 in DMOMA and Y = H5 in OMA) . The numbering used for the molecules is shown in Fig. 1.

RESULTS AND DISCUSSION

Methylaminomethanol (MMA)

The nine initial MMA conformations yielded seven conformers, the geom- etries and relative and absolute energies of which are listed in Table 1. The lowering of energy when a polar bond is trans to an .sp3 lone pair (the “ano- merit effect”) [3,15] explains why the most stable conformers are MM and GM, in both of which two polar bonds (C-O and C-N) are so arranged, and why the least stable are AA and AG, in which the only bonds trans to the lone pairs are C-H, for which anomeric interaction is much weaker than for more polar bonds.

The following geometric trends can likewise be explained as anomeric interactions.

(i) The C-O and C2-N bonds are longest when anti to the lone pairs on N (in GM, MM and AM) and 0 (in GM, MM, MA and GG), respectively.

(ii) In all the conformers, the C-H10 bond is longer than C-H11 and C-H9; the lengths of these bonds in GM, for example, are 1.0882,1.0829 and 1.0808 A, respectively.

(iii) The anomeric effects of N and 0 are roughly additive as regards the length of C-H7, which is longest when trams to both the lone pairs (1.0925 A, in AA), shortest when trans to neither (1.0766 A, in MM) and has interme- diate lengths when trans to just one (to an 0 pair in GM, AM, GG and AG, and to the N pair in MA). The same holds for C-H6, which is shortest (1.076 A) in GM, longest (1.0923 A) in AG and has intermediate lengths in the other conformers. For a given conformer, the anomeric position of one of these bonds always makes it longer than the othey in MM, for example, C-H6 is longer than C-H7 (1.0818 as against 1.0766 A) because of the anomeric effect of an 0 lone pair.

(iv) The angle N-C-O is widest when C-O is anti to the N lone pair and N- C to an 0 pair (115.47” in GM and 115.08” in MM), narrowest when neither bond is in anomeric position (105.58” in AA and 107.47” in AG), and has intermediate values when an anomeric effect is exerted only by N (in AM) or only by 0 (in MA and GG).

(v) Because of the lone pair on N, N-C-H10 is wider than N-C-H9 and N- C-H11 in all the conformers.

(vi) A lone pair on 0 makes O-C-H7 wider in GM, AM, AA, GG and AG

TA

BL

E

1

Geo

met

ries

an

d en

ergi

es o

f th

e co

nfo

rmer

s of

met

hyl

amin

omet

han

ol e

ner

gies

in k

cal

mol

-’

Fea

ture

G

M

MM

A

M

MA

A

A

GG

A

G

Bon

d le

ngth

s (A

) c2

-01

N3-

C2

C4-

N3

H5-

01

H6-

C2

H7-

C2

H8-

N3

H9-

C4

HlO

-C4

Hll

-C4

1.45

22

1.45

39

1.45

66

1.43

70

1.44

16

1.44

05

1.44

74

1.44

22

1.43

95

1.43

46

1.45

56

1.45

12

1.45

01

1.43

63

1.47

00

1.47

07

1.47

49

1.47

00

1.46

66

1.46

96

1.46

34

0.96

31

0.96

41

0.96

36

0.96

47

0.96

26

0.96

47

0.96

32

1.07

60

1.08

18

1.08

19

1.08

34

1.08

40

1.08

32

1.09

23

1.08

22

1.07

66

1.08

24

1.08

32

1.09

25

1.08

28

1.08

26

1.00

09

1.00

06

1.00

17

1.00

18

1.00

09

0.99

91

0.99

68

1.08

08

1.08

12

1.08

11

1.08

08

1.08

05

1.08

12

1.08

12

1.08

82

1.08

46

1.08

31

1.08

86

1.09

04

1.08

88

1.09

07

1.08

29

1.08

21

1.08

26

1.08

22

1.08

29

1.07

96

1.07

84

Bon

d an

gles

(“)

N

3-C

2-01

C

4-N

3-C

2 H

5-O

l-C

2 H

6-C

2-01

H

6-C

2-N

3 H

7-C

2-01

H

7-C

2-N

3 H

7-C

2-H

6 H

8-N

3-C

2 H

8-N

3-C

4 H

9-C

4-N

3 H

lO-C

4-N

3 H

lO-C

4-H

9 H

ll-C

4-N

3 H

ll-C

4-H

9 H

ll-C

I-H

I0

115.

47

115.

08

109.

92

109.

63

105.

58

109.

97

107.

47

116.

95

116.

01

115.

08

116.

43

115.

10

115.

67

116.

61

110.

98

110.

58

111.

26

108.

09

110.

99

107.

96

110.

44

104.

38

110.

22

109.

92

111.

42

111.

63

104.

58

108.

64

109.

19

108.

73

109.

11

107.

68

109.

03

113.

86

112.

90

109.

98

103.

98

109.

92

105.

12

109.

65

111.

06

110.

74

108.

30

109.

25

108.

74

114.

35

112.

79

108.

29

108.

69

109.

37

109.

41

109.

20

108.

69

108.

20

109.

08

108.

43

112.

71

114.

46

111.

80

112.

04

110.

69

114.

33

114.

96

112.

90

113.

53

112.

12

113.

39

114.

52

114.

50

116.

02

109.

16

109.

16

108.

86

109.

00

109.

03

109.

53

109.

31

113.

85

113.

08

112.

83

113.

83

i’113

.75

113.

18

113.

30

108.

27

109.

32

109.

50

108.

47

108.

59

108.

87

108.

65

108.

88

108.

66

108.

66

109.

00

109.

20

108.

01

108.

03

107.

99

108.

26

108.

14

108.

09

107.

94

108.

74

108.

68

108.

53

108.

24

108.

75

108.

30

108.

17

108.

42

108.

77

Tor

sion

al

angl

es (

“)

N3-

C2-

Ol-

H5

H6-

C2-

Ol-

H5

H7-

C2-

Ol-

H5

C4-

N3-

C2-

01

C4-

N3-

C2-

H6

C4-

N3-

C2-

H7

H&

N3-

C2-

01

H8-

N3-

C2-

H6

H8-

N3-

C2-

H7

H9-

C4-

N3-

C2

H9-

C4-

N3-

H8

HlO

-C4-

N3-

C2

HlO

-C4-

N3-

H8

Hll

-C4-

N3-

C2

Hll

-C4-

N3-

H8

- 68

.52

171.

62

54.3

8 68

.30

- 17

4.50

-

55.4

8 -

65.0

2 52

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171.

19

173.

13

- 53

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- 65

.76

67.4

7 t 5

5.46

-

171.

30

63.9

7 -

174.

01

- 59

.42

65.8

8 -

176.

59

- 54

.34

57.6

0 62

.28

- 17

8.23

-

177.

12

- 58

.88

-58.

10

- 77

.64

-67.

14

46.5

3 53

.46

165.

88

172.

48

175.

98

177.

62

- 48

.37

-53.

12

-62.

10

-60.

61

73.5

5 68

.65

58.1

1 60

.06

- 16

6.25

-

170.

67

46.3

7 17

8.64

-4

5.76

-

72.7

2 60

.30

- 16

8.39

16

9.73

-

59.5

9 74

.10

- 16

4.64

17

0.73

-

70.9

6 -

43.2

6 -

69.2

0 46

.01

77.6

3 51

.01

167.

52

62.5

6 38

.91

152.

80

- 17

6.06

15

8.97

-9

0.23

-5

5.17

-

80.8

1 31

.27

176.

36

179.

84

170.

09

- 51

.46

-50.

18

- 53

.74

- 62

.41

-58.

81

- 68

.24

69.7

7 71

.17

67.9

3 58

.60

62.1

2 51

.81

- 16

9.22

-

167.

90

- 17

2.02

162.

58

40.1

3 -

78.8

5 -

66.4

2 53

.35

173.

69

152.

83

- 87

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32.9

4 17

3.00

-

46.6

7 -

65.6

7 74

.66

54.9

0 -

164.

77

Ene

rgie

s (k

cal

mol

-‘)

Rel

ativ

e 0.

37

0.00

1.

29

1.64

7.

02

0.73

6.

62

Abs

olu

te

- 13

0910

.383

5 -

1309

10.7

551

- 13

0909

.464

8 -

1309

09.1

099

- 13

0903

.733

8 -

1309

10.0

262

- 13

0904

.131

3

240

than in MM and MA, and makes O-C-H6 wider in MM, AM, MA, AA and AG than in GM and GG. For a given conformer, the anomerically arranged angle is always wider than the other; in GM, for example, O-C-H7 is 109.98” and O-C-H6 104.38”.

(vii) N-C-H7 and N-C-H6 are widest when the N lone pair is truns to C- H7 (in MA and AA) and C-H6 (in GG and AG), respectively. For a given conformer, the angle whose C-H bond is truns to the N lone pair is always wider than the other; in MA, for example, N-C-H7 is 114.35” and N-C-H6 107.68”.

l-Methoxymethylumine (OMA)

The five initial OMA conformations converged on the three conformers GG (the most stable), AG and AM. Their geometries and absolute and relative energies are listed in Table 2. The relative energies are explicable in terms of the anomeric effect (GG has two polar bonds truns to lone pairs while AM, the least stable conformer, has only C-H bonds), as are the following geometric patterns.

(i) C2-0 is longer in AG and GG, in which it is truns to the N lone pair, than in AM.

(ii) C-H7 is longest in AM, in which it is truns to both 0 and N lone pairs and longer than the non-anomerically oriented C-H8 bond (1.0921 as against 1.0842 A). C-H8 is longer in AG and AM, in which it is truns to an 0 lone pair, than in GG, in which C-H7 is truns to an 0 lone pair and longer than C-H8.

(iii) Because of the truns 0 lone pair, C-H9 and C-H10 are longer in all three conformers than C-Hll, which is never truns to a lone pair.

(iv) The angle N-C-O is widest in GG, in which both N and 0 exert ano- merit effects, and narrowest in AM, in which neither atom exerts such an effect.

(v) H9-C-0 and HlO-C-O are wider in all three conformers than Hll-C- 0, for the same reason as mentioned in point (iii) above.

(vi) The effect of the N lone pair makes H7-C-N wider than H8-C-N in AM.

(vii) H8-C-0 is wider in AM and AG, in which it is influenced by a lone pair on 0, than in GG, in which H7-C-0 is influenced by a lone pair on 0 and is wider than H8-C-0.

I-Methoxy-N,N-dimethyl-methylamine (DMOMA)

The geometries and absolute and relative energies of the three DMOMA conformers found (AM, AG and GA) are listed in Table 3. The relative ener- gies are again explained by anomeric interactions, which likewise account for the following trends.

241

TABLE 2

Geometries and energies of the conformers of l-methoxymethylamine

Feature AM AG GG

Bond lengths (A) C2-Nl 03-C2 c4-03 H5-Nl H6-Nl H7-C2 H8-C2 H9-C4 HlO-C4 Hll-C4

Bond angles (“) 03-C2-Nl C4-03-C2 H5-Nl-C2 H6-Nl-C2 H6-Nl-H5 H7-C2-Nl H7-C2-03 H8-C2-Nl H8-C2-03 H8-C2-H7 H9-C4-03 HlO-C4-03 HlO-C4-H9 Hll-C4-03 Hll-C4-H9 Hll-CI-HlO

Torsional angles (“) 03-C2-Nl-H5 03-C2-Nl-H6 H7-C2-Nl-H5 H7-C2-Nl-H6 H8-C2-Nl-H5 H8-C2-Nl-H6 C4-03-C2-Nl C4-03-C2-H7 C4-03-C2-H8 H9-C4-03-C2 HlO-C4-03-C2 Hll-C4-03-C2

1.4473 1.4376 1.4402 1.4366 1.4498 1.4481 1.4403 1.4398 1.4390 0.9985 1.0011 0.9987 0.9980 1.0011 0.9980 1.0921 1.0826 1.0825 1.0842 1.0826 1.0765 1.0853 1.0854 1.0835 1.0846 1.0854 1.0860 1.0781 1.0785 1.0785

106.48 110.63 115.92 114.14 114.26 115.13 113.74 112.99 115.13 115.48 112.99 116.68 114.07 111.00 113.11 114.28 109.32 108.40 108.32 109.38 109.29 108.91 109.32 109.43 110.85 109.38 104.11 108.02 108.80 109153 111.38 111.35 111.58 111.23 111.35 111.22 108.94 108.77 108.94 106.43 106.77 106.63 109.41 109.28 109.16 109.39 109.28 109.26

55.20 - 63.54 -60.38 - 170.11 63.54 75.74

- 64.34 175.95 176.34 70.34 - 56.96 -47.54

174.78 56.96 56.93 - 50.54 - 175.95 - 166.94

- 178.42 - 180.00 - 65.91 - 55.08 - 59.53 56.90

63.26 59.53 173.85 59.42 60.79 64.35

- 62.32 -60.79 -57.51 178.60 180.00 - 176.54

Energies (kcal mol -I) Relative Absolute

7.26 - 130904.0272

0.70 0.00 - 130910.5904 - 130911.2893

242

TABLE 3

Geometries and energies of the conformers of 1-methoxy-NJ-dimethylmethylamine

Feature AM AG GA

Bond lengths (A) C2-Nl 03-C2 c4-03 C5-Nl C6-Nl H7-C2 H&C2 H9-C4 HlO-C4 Hll-C4 H12-C5 H13-C5 H14-C5 H15-C6 H16-C6 H17-C6

Bond angles (“) 03-C2-Nl C4-03-C2 C5-Nl-C2 C6-Nl-C2 C6-Nl-C5 H7-C2-Nl H7-C2-03 H8-C2-Nl H8-C2-03 H8-C2-H7 H9-C4-03 HlO-C4-03 HlO-C4-H9 Hll-C4-03 Hll-C4-H9 Hll-C4-HlO H12-C5-Nl H13-C5-Nl H13-C5-H12 H14-C5-Nl H14-C5-H12 H14-C5-H13 H15-C6-Nl H16-C6-Nl H16-C6-H15 H17-C6-Nl H17-C6-H15 H17-C6-H16

1.4496 1.4343 1.4568 1.4358 1.4506 1.4314 1.4405 1.4388 1.4463 1.4688 1.4725 1.4670 1.4666 1.4726 1.4696 1.0945 1.0835 1.0842 1.0843 1.0835 1.0856 1.0850 1.0854 1.0788 1.0846 1.0854 1.0850 1.0780 1.0786 1.0786 1.0918 1.0849 1.0905 1.0815 1.0817 1.0814 1.0765 1.0823 1.0823 1.0824 1.0823 1.0784 1.0810 1.0817 1.0817 1.0913 1.0849 1.0907

108.04 110.18 111.12 114.18 114.46 114.24 113.64 113.72 114.34 113.19 113.66 113.07 114.01 113.00 113.89 112.51 109.44 108.10 108.85 109.29 110.59 108.71 109.45 112.95 110.43 109.30 104.85 108.31 109.15 109.22 111.31 111.33 110.80 111.21 111.32 110.53 108.98 108.77 109.29 106.42 106.82 106.22 109.47 109.28 110.48 109.39 109.28 109.48 112.42 112.28 113.01 108.93 108.89 109.00 108.60 109.26 108.26 108.41 109.16 109.59 109.20 108.83 108.59 109.24 108.34 108.27 109.58 109.15 108.43 108.88 108.89 109.38 108.26 108.34 109.20 113.07 112.27 112.30 108.61 108.84 108.80 108.32 109.27 108.68

243

TABLE 3 (continued)

Feature AM AG GA

Torsional angles (’ ‘) 03-C2-Nl-C5 58.09 03-C2-Nl-C6 - 169.78 H7-C2-Nl-C5 - 62.09 H7-C2-Nl-C6 70.03 H8-C2-Nl-C5 177.95 H8-C2-Nl-C6 - 49.93 C4-03-C2-Nl - 176.69 C4-03-C2-H7 -54.24 C4-03-C2-H8 64.54 H9-C4-03-C2 59.63 HlO-C4-03-C2 -62.11 Hll-C4-03-C2 178.83 H12-C5-Nl-C2 69.30 H12-C5-Nl-C6 - 62.43 H13-C5-Nl-C2 - 170.28 H13-C5-Nl-C6 57.99 H14-C5-Nl-C2 -51.51 H14-C5-Nl-C6 176.76 H15-C6-Nl-C2 54.33 H15-C6-Nl-C5 - 173.72 H16-C6-Nl-C2 172.56 H16-C6-Nl-C5 - 55.49 H17-C6-Nl-C2 - 66.98 H17-C6-Nl-C5 64.96

- 65.73 163.73 65.45 - 63.75

174.08 42.21 -54.74 174.73

54.50 - 78.75 - 174.33 53.76 - 179.88 - 64.93 - 59.59 55.12

59.81 172.73 60.78 57.49

-60.77 -63.82 - 179.99 177.51

67.79 68.00 - 63.72 -64.13

- 171.08 - 171.59 57.41 56.28

- 52.98 - 53.26 175.51 174.61 53.15 48.02

- 175.32 - 179.24 171.24 167.02

-57.22 - 60.25 -67.62 -72.23

63.91 60.51

Energies (kcal mol -‘) Relative Absolute

4.86 0.00 0.50 - 179786.4124 - 179791.2716 - 179790.7667

(i) C2-0 is longest in AG, in which it is truns to the N lone pair, and C2-N is longest in GA, in which it is tram to an 0 lone pair.

(ii) Because of the anomeric effect of an 0 lone pair, C-H9 and C-H10 are longer than C-H11 in all three conformers.

(iii) Because of the N lone pair, in all three conformers C-H12 is longer than C-H13 and C-H14, and C-H17 is longer than C-H15 and C-H16.

(iv) C-H7 is longest in AM, in which it is influenced by the lone pairs of both N and 0 and is longer than C-H& which is only influenced by 0.

(v) The angle N-C-O is narrowest in AM, in which it is not influenced by any anomeric effect.

(vi) Because of the anomeric effect of 0 on O-C-H9 and 0-C-HlO, both these angles are wider than O-C-H11 in all three conformers.

(vii ) O-C-H8 is least in the conformer in which it is not subject to anomeric effect, GA, in which it is narrower than the anomerically arranged O-C-H7.

244

(viii) Because of the N lone pair, in all three conformers HlB-C-N is wider than HlS-C-N and H14-C-N, and H17-C-N is wider than HE-C-N and HlG-C-N.

(ix) Because of the N lone pair, N-C-H7 is widest in AM and N-C-H8 is widest in GA. For a given conformer, the wider of these two angles is the one subject to anomeric influence; in GA, for example, N-C-H8 is 112.95 ’ and N- C-H7 108.10”.

ACKNOWLEDGEMENTS

This work was supported by the Xunta de Galicia and through the award of a grant to B.F.

REFERENCES

1 2

3

10

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