Journal of Molecular Structure (Theo&em), 209 (1990) 201-209 Elsevier Science Publishers B.V.. Amsterdam

201

AB INITIO STUDIES OF MOLECULES WITH N-C-O UNITS PART IV. N,N-DIMETHYLAMINOMETHANOL, 2-AMINO-2- PROPANOL AND l-METHYLAMINOETHANOL

LUGS CARBALLEIRA, BERTA FERNANDEZ* and MIGUEL A. RIOS

Departamento de Quimica Fkica, Universidad de Santiago de Compostela, E-15706 (Spain)

(Received 7 August 1989; in final form 13 November 1989)

ABSTRACT

We have carried out an ab initio conformational 4-21G study with full optimization of the geometry of N,N-dimethylaminomethanol, 2-amino-2-propanol and l-methylaminoethanol, all of which have the structural N-C-O unit. The results are interpreted in terms of the anomeric effect and compared with those already available for molecules including the N-C-O unit.

INTRODUCTION

We have carried out an ab initio conformational study of N,N-dimethylam- inomethanol, 2-amino-2-propanol and 1-methylaminoethanol in order to ob- tain a molecular-mechanics force field for compounds with the N-C-O unit and to complement the results found for other molecules containing this struc- tural unit [l-3].

There are few experimental data available for the molecules studied and only a 13C NMR study of 2-amino-2-propanol [4] is probably worth mentioning in this respect.

This work, like the preceding studies, was carried out using the 4-21G basis set [ 51, Pulay’s method [ 61 and the program TEXAS [ 71, starting from all the staggered conformations (nine for l-methylaminoethanol and five for the other two compounds). Unconstrained optimization was applied in all cases until the Cartesian components of the forces acting on the atoms fell below 0.001 mdyn.

The conformers obtained were named with letters denoting the main torsion angles (A = 180’) G= - 60’ and M= 60 o ) , with that of the angle H-O-C-N appearing first, followed by that of the angle X-N-C-O, where X denotes the carbon in 1-methylaminoethanol and the N lone pair in the other two com- pounds. The numbering scheme used is shown in Fig. 1.

*To whom correspondence should be addressed.

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

202

(1)

,A01 \ % ’ c2’ \

B

\ \ \ /’

\ ,’ \

Hg ,t5++13 % d

3’

i/

%I

%

51 131

Fig. 1. The atom numbering used for NJ/-dimethylaminomethanol ( 1 ), Z-amino-2-propanol(2 ) and 1-methylaminoethanol(3).

The results obtained were interpreted through the anomeric effect [ 8,9]. This was first detected in carbohydrates and is based on the favourable inter- action resulting from the establishment of a tram arrangement between a lone pair and the Y atom of a neighbouring polar bond (Lp-X-C (sp3)-Y). Its oc- currence influences the stability (which increases with the number of inter- actions of this type), the structure (which increases the length of the C(sp3)- Y bonds and the width of the X-C(sp3)-Y angles) and the reactivity of the compound.

Finally, we compared the results obtained in this study with others already available for analogous compounds [l-3,8].

203

RESULTS AND DISCUSSION

This compound has five conformers, namely AA, AG, GA, GM and GG, the main geometrical features and relative energies of which are listed in Table 1.

TABLE 1

Main geometrical features and relative energies of the conformers of N,N-dimethylaminomethanol

Parameter Conformer

AA AG GA GM GG

Bond lengths (A) c2-01 N3-C2 C5-N3 H7-C2 H&C2 H9-C5 HlO-C5 Hll-C5 H12-C4 H13-C4 H14-C4

Bond angles (“) N3-C2-01 C4-N3-C2 H7-C2-01 H7-C2-N3 H8-C2-01 H8-C2-N3 H9-C5-N3 HlO-C5-N3 Hll-C5-N3 H12-C4-N3 H13-C4-N3 H14-C4-N3

Torsional angles (“) N3-C2-Ol-H6 C4-N3-C2-01 C5-N3-C2-01

Relative energy (kcal mol-‘)

1.4567 1.4432 1.4529 1.4381 1.4366 1.4343 1.4478 1.4429 1.4560 1.4594 1.4728 1.4659 1.4691 1.4707 1.4678 1.0823 1.0827 1.0764 1.0843 1.0772 1.0823 1.0936 1.0821 1.0829 1.0928 1.0817 1.0810 1.0813 1.0816 1.0810 1.0848 1.0913 1.0898 1.0903 1.0913 1.0823 1.0824 1.0827 1.0785 1.0821 1.0817 1.0814 1.0817 1.0814 1.0809 1.0823 1.0765 1.0816 1.0822 1.0798 1.0848 1.0920 1.0860 1.0901 1.0925

109.68 107.61 115.00 110.04 112.61 113.75 113.85 113.95 114.62 114.26 109.89 110.72 104.31 105.06 105.94 109.02 108.39 109.13 113.52 108.24 109.89 109.44 110.17 111.02 109.63 109.02 112.23 108.58 108.14 111.45 108.86 108.90 109.09 109.35 109.08 112.29 113.09 113.17 112.26 113.08 109.17 109.57 109.41 108.46 109.34 108.86 108.96 109.10 108.95 109.06 109.17 108.38 109.11 109.59 109.12 112.29 112.43 112.38 112.95 112.95

- 179.96 - 65.61

65.61

- 164.07 58.79

- 168.39

- 69.05 -49.31 -81.62 -61.30 162.26 58.77

72.42 - 64.97 - 169.38

1.23 5.95 0.00 0.40 5.67

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The highest stability of the GA conformer arises from its anomeric effect, which involves more polar bonds (C-N and C-O) than in the other four conformers. The anomeric effect is also responsible for the following structural trends.

(1) The C2-01 bond is longer in AA and GA than it is in the other three, as such a bond is in the anti configuration with respect to the nitrogen lone pair in these two.

(2) The C2-H8 bond is longest in GG and AG (1.0928 and 1.0936 A, respec- tively) ss a result of accumulated anomeric effects: this bond is in the anti configuration with respect not only to an oxygen lone pair but also that of the nitrogen. The C2-H7 bond is longer in AA, AG and GM than in the other two conformers because of one of the oxygen lone pairs in the first two and of the nitrogen lone pair in GM. The C2-H8 bond is longer than the C2-H7 bond in GA, GG and AG, because in the first two conformers the C2-H7 bond is not involved in anomeric interactions and in AG it is only subject to the action of the oxygen.

(3) The C4-H14 bond is longer than C4-H12 and C4-H13 because in every case it is in a trans position with respect to the nitrogen lone pair. This is also the case for C5-HlO compared with C&Ii9 and C5-Hll.

(4) The N-C-O angle is widest in GA (115.00’ ), where it is in a favourable arrangement with respect to one of the oxygen lone pairs and the nitrogen lone pair, and narrowest in AG (107.61’ ) where no anomeric effect favouring its opening occurs.

(5) As the C4-H14 bond is anti with respect to the nitrogen lone pair, the N-C4-H14 angle is wider than N-C4-H13 and N-C4-H12 in every case. Like- wise, the N-C5-HlO angle is wider than N-C5-H9 and N-C5-Hll.

(6) The N-C2-H8 angle is wider in GG and AG and N-C2-H7 is widest in GM because of the nitrogen; in any of these three conformers the angle of these two subject to the anomeric effect is also the widest (e.g. 108.14” and 113.52”, respectively, in GM).

(7) The O-C2-H7 angle is wider in AA and AG, where it is in a favourable arrangement with respect to an oxygen pair. This angle is narrower than 0-C2-H8 in GA, GM and GG, also as a result of its arrangement with respect to the oxygen.

The consequences of the replacement of the atom H6 with a methyl group [ 11 can be summarized as follows. (1) The bonds most significantly affected are 0-C2, H7-C2 and H8-C2, with a maximum difference of 7.4 x 10B3 A. (2 ) The bond angles are very similar in conformers AA and AG, with differences smaller than 0.6 o in every case. The N-C-O angle in conformer GM undergoes a greater deviation (1.08” ) because of its hydrogen bond, the strength of which increases as the N-C-O angle becomes narrower. (3) The torsion angles also reflect the occurrence of the above-mentioned interaction. Thus, the H-O-C-N angle is much narrower than C-O-C-N in l-methoxy-N&V-dime- thylamine. No significant differences, however, were found in the torsional angles between the AA conformers.

205

The replacement of’the methyl groups by one (methylaminoethanol [l] )

and two (aminomethanol [ 81) hydrogen atoms gives rise to the following geo- metric effects. (1) The greatest differences in length are those of the C2-N (maximum variation 0.0128 A) and C5-N bonds in substituting C4 by a hy- drogen atom (4.6 x 10m3 A). The differences are insignificant (of the order of 10V3 A) for the six GM and AA conformers. (2) The angle most significantly affected in passing from N,N-dimethylaminomethanol to methylaminoe- than01 is C-N-C, which increases by up to 3” in conformer GA. The angle C- N-H varies similarly in passing from methylaminomethanol to aminome- thanol, being up to 1.92” wider in the latter. (3) As far as the main torsional angles are concerned, the greatest angular difference in passing to the primary amine compound corresponds to the H-O-C-N angle in the conformer AG (up to a difference of 13.94’ ).

2-Amino-2-propanol

The five conformations studied gave three conformers, AA, GA and GM, the energies and main geometrical features of which are gathered in Table 2. The

TABLE 2

Main geometrical features and relative energies of the conformers of 2-amino-2-propanol

Parameter Conformer

AA GA GM

Bond lengths (A) c2-01 N3-C2 C6-C2 C7-C2

1.4645 1.4618 1.4469 1.4476 1.4510 1.4642 1.5310 1.5313 1.5319 1.5310 1.5261 1.5311

Bond angles (“) N3-C2-01 C6-C2-01 C6-C2-N3 C7-C2-01 C7-C2-N3

107.50 112.88 107.44 109.30 109.37 109.81 109.20 108.88 108.58 109.30 103.94 104.79 109.20 109.45 113.94

Torsional angles (“) N3-C2-Ol-H8 H4-N3-C2-01 H5-N3-C2-01

- 180.00 -61.51 -41.30 62.13 73.65 - 69.50

-62.13 -58.15 159.00

Relative energy (kcal mol-‘) 0.63 0.00 0.46

206

differences in stability can be roughly interpreted on the basis of the anomeric effect. Thus, the most stable conformer is GA, where this effect involves the most polar bonds. However, the other two conformers are only less than 1 kcal mol-’ less stable than this, which is an insignificant difference in these calculations.

The geometric analyst ~rformed plowed us to detect the follo~ng trends arising from the anomeric effect.

(1) The length of the C2-01 bond is longer in the conformers AA and GA, where it is anti with respect to the nitrogen lone pair.

(2 ) The length of the C2-N3 bond is increased in GA and GM as a result of its favourable arrangement with respect to the oxygen atom.

(3) The C2-C7 bond is longer in AA and GM than it is in GA as a result of being favoured by an oxygen lone pair in AA and by the nitrogen ltne pair in GM. In GA the C2-C6 bond is longer than C2-C7 (1.5313 vs. 1.5261 A) because of the effect of the oxygen.

(4) The O-C-N angle is widest in GA on account of the accumulation of anomeric effects.

(5) The N-C2-C7 angle is wider in GM than it is in the other conformers as its C2-C? bond is in a favourable arrangement with respect to the nitrogen lone pair. For the same reason, this angle is also wider than N-C2-C6 in the same conformer.

(6) The 0-C2-C7 angle is widest in conformer AA because of its arrauge- ment with respect to the oxygen atom. In addition, it is narrower than 0-C2- C6 in GA and GM, where the 0-C2-C6 is increased by the effect of the oxygen atom.

The analysis of the evolution of the geometry of each conformer in the com- pounds studied in this series of papers [l-3,8], reveals the following trends. (1) The bond lengths that undergo the greatest v~iation are C-O and C-N (maximum variation 5.7 x low3 and 7.8 x 10T3 A, respectively). Both the bond distances and the bond angles in the AA conformer of 1-aminoethanol are intermediate between those of the same conformer in 2-aminopropanol and aminomethanol. (2) The angles that experience the most significant changes are O-C-N and C-C-N in passing from 2-amino-2-propanol to l-amino- ethanol (about 1.2” wider in the latter). (3) The main torsional angles hardly vary in the AA conformers, and the H-N-C-O angles of the GA and GM con- formers vary by only 4.11” at most.

The study conducted on this system led to seven conformers (AA, AM, AG, MA, MM, GM and GG) for the nine starting conformations. Their corre- sponding relative energies and main geometrical features are given in Table 3. As can be seen, the most stable conformers are MM and GM, which involve

207

TABLE 3

Main geometrical features and relative energies of the conformers of 1-methykuninomethanol

Parameter Conformer

AA AM AG MA MM GM GG

Bond lengths (ii) C2-01 - N3-C2 C5-C2 H&-C2 HQ-C4 HlO-C4 Hll-c4

1.4483 1.4609 1.4483 1.4427 1.4582 1.4565 1.4433 1.4611 1.4391 1.4481 1.4650 1.4437 1.4469 1.4579 1.5288 1.5281 1.5408 1.5284 1.5286 1.5225 : .5295 1.0881 1.0824 1.0832 1.0838 1.0770 1.0824 1.0834 1.0809 1.0812 1.0816 1.0809 1.0813 1.0809 1.0816 1.0896 1.0829 1.0888 1.0892 1.0844 1.0885 1.0869 1.0825 1.0825 1.0774 1.0800 1.0821 1.0828 1.0796

Bond angles (“) N3-C2-01 c5-C2-01 C5-C2-N3 H8-CZ-01 H8-C2-N3 HQ-C4-N3 HlO-C4-N3 Hll-C4-N3

104.45 108.70 105.86 106.99 113.75 114.11 108.68 111.38 110.04 109.63 110.65 110.37 105.04 105.51 110.86 110.61 114.85 111.55 110.35 110.57 116.39 108.93 109.01 109.99 104.36 103.15 109.13 110.15 111.02 107.90 107.18 112.94 108.29 107.34 106.48 108.88 108.80 108.72 108.49 109.07 109.11 109.06 113.58 112.80 114.20 113.51 113.06 113.94 113.98 109.65 108.74 107.84 109.90 108.77 108.96 107.98

Torsional angles (“) N3-CZ-Ol-H6 160.23 - 173.02 C4-N3-C2-01 143.33 61.70 H7-N3-CZ-01 12.29 - 66.89

Relative energy (kcal mol-‘) 6.03 1.11

171.93 53.37 65.64 - 65.57 - 45.55 - 57.88 177.93 56.00 67.18 -66.16 162.49 49.12 - 78.26 -65.61 156.38

7.31 2.53 0.00 0.49 1.51

anomeric ~~ractions with C-N and C-O bonds. The different geometries re- veal the following anomeric trends.

(1) The CZ-01 bond is longer in AM, MM and GM than it is in the other conformers as this bond is in a more favourable arrangement with respect to the nitrogen atom in these three.

(2) The lengths of the C2-C5 bonds reflect the additivity of the anomeric effect. Thus, this bond is shortest (1.5225 A) in GM, in which it is not subject to anomeric interactions, and longest in AG (1.5408 A) which involves ano- merit interactions with the nitrogen and oxygen lone pairs.

(3 ) The aforementioned additivity of anomeric effects is also reflected in the lengths of the C2-H8 bond which is shortest in MM (1.0770 A) and longest in AA (1.0881 A).

(4) The C4-HlO bond is longer than C4-H9 and C4-Hll in all conformers

208

on account of the arrangement of the former, which favours the anomeric in- teraction. For the same reason, the N-C4-HlO angle is wider than N-C4-H9 and N-C4-Hll.

(5) The N-CZ-H8 angle is widest in AA and MA because of the effect of the nitrogen.

(6) The N-C2-C5 angle is widest in AG and GG because of the effect of the nitrogen.

(7) The 0-CZ-C5 angle is narrower in conformers GM and GG, where the C2-C5 bond is not anti with respect to any lone pair. A similar behaviour is reflected by the values of the 0-C2-H8 angle in conformers MA and MM, in which it is not involved in anomeric interactions.

(8) The O-C-N angle again reflects the additivity of the anomeric effects; thus, it is narrowest in AA and AG (104.45’ and 105.86”, respectively) where the anomeric effect is not favoured, and widest in MM and GM (113.75” and 114.11’) respectively) which foster the interaction with both the oxygen and the nitrogen.

The geometric results obtained for conformer GM were compared with those found previously for the corresponding conformers of 1-aminoethanol [ 31 and aminoethanol [ 8 1. The differences can be summarized as follows. (1) The bond ~stances undergoing the greatest changes in replacing C4 for a hydrogen atom are those of C-C and C2-H (0.0058 and 0.0064 A, respectively). This change is also reflected in passing from I-methylaminoethanol to aminome- thanol. In this process, the C2-0 and C2-N bonds are also shortened by about 4 x 10B3 A. (2) The greatest variation in bond angles corresponds to the O-C- C angle with 5.29“ as the maxims difference in passing from l-methylami- noethanol to the primary amine. (3) The torsional angle H7-N-C-O suffers the biggest change. It is wider than -60” in l-methylaminoethanol and nar- rows to -57.11” in 1-aminoethanol, to favour the interaction between the oxygen and H7.

This work was supported by the Xunta de Galicia, and by a Spanish Ministry of Education and Science F.P.I. grant awarded to B.F.

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