An ab initio conformational analysis of isobutylamine and diisopropylamine

Journal of Molecular Structure (Theo&em), 251 (1991) 319-326 Elsevier Science Publishers B.V., Amsterdam

319

An ab initio conformational analysis of isobutylamine and diisopropylamine Berta Femdndez’, Ricardo A. Mosquera and Miguel A. Rios Departamento de Quimica Fkica, Facultad de Quimica, Santiago de Composteh, E-15706 (Spain) (Received 19 April 1991)

Abstract A 4-21G conformational analysis with complete geometry optimization was carried out for isobutyhunine and diisopropylamine. The results are compared with available geometrical data for similar amines in order to establish general conformational trends.

INTRODUCTION

Isobutylamine (IBA) has great importance as a reagent used, for example, in the preparation of antiglaucoma agents, leukotriene inhibitors and antitumour agents, and in the treatment of waste water. Its structure has been investigated experimentally by IR, 15NNMR, 13CNMR, mass and Raman spectroscopies [l-3], and in theoretical studies [ 41. Diisopropylamine (DIPA) has been the subject of a gas electron diffraction study [5], in which just one conformer was found. In this work we studied all possible conformations of IBA and DIPA by means of ab initio calculations using Pulay’s method [ 61 and the 4-21G basis set [ 71. Starting from all the staggered conformations, optimization was carried out until the Cartesian components of the residual forces on the atoms were smaller than 0.001 mdyn. The non-planarity at the nitrogen atom was measured as the distance between the N atom and the plane formed by its three neighbours. The results are compared with previously published data for DIPA, IBA and similar amines, and discussed in terms of anomeric interactions between the nitrogen lone pair (Lp) and the polar C-Y bonds anti to it in Lp-X-C-Y; this interaction increases the stability, the C-Y bond length and the X-C-Y angle

PI. In this paper, conformers are named with two letters representing the approximate values of the main torsional angles (A, 180”; G, 60”; M, - 60” ). ‘Author to whom correspondence should be addressed.

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0 1991 Elsevier Science Publishers B.V. All rights reserved.

320

DI PA

Fig. 1. Atom numbering of IBA and DIPA,

The first letter correspondsto Lp-N-C-C in IBA and to HS-C-N-C in DIPA, and the second to H-C-C-N in IBA and to HlG-C-N-C in DIPA. The atomic numberingis shown in Fig. 1. RESULTS AND DISCUSSION

Isobutylumine (IBA) Numerically,the most stable IBA conformers are AG and MM (Table l), but the others differ from them by less than 1 kcal mol-‘, which is not significant in this kind of calculation. The propylamine conformers identified by Allingerand co-workers,using the 6-31G* basis set, also differed in energyby less than 1 kcal mol-’ 193. Table 2 showsthe IBA geometricalfeaturesrelated to anomericinteractions. The C3-C2 bond is longer in AA and AG than in the other conformersbecause it is anti to the nitrogen lone pair. For the same reason, the H9-C2 bond is longestin MG and MM. The anomericinfraction also makesthe C-C-N bond angle wider in AA and AG than in the other confo~e~ (116.3” and 115.8” respectivelyvs. 111.0” in the others), and H9-C2-Nl widest in MG and MM (about 5 0 wider than in AA, MA and AG) . Table 3 lists the other main bond lengths and bond angles of IBA together

321 TABLE 1 Absolute and relative energies, dipole momenta, maximum absolute value of the cartesian components of the residual forces and main torsional angles of the conformers of IDA and DIPA Conformer

Abs. E (au)

Rel. E (kcai mol-‘1

‘;DJ

IFI

Torsional angle (deg)

(m&r)

IBA AA AG MA MG MM

-211.8673883 -211.8679155 - 211.8668746 -211.8665575 -211.8679558

0.36 0.02 0.68 0.88 0.00

1.369 1.425 1.425 1.299 1.421

0.000928 0.000816 0.000706 0.000388 0.000281

H7-N-C-C/ HB-N-C-C -65.5165.5 -60.2169.6 - 155.3176.2 - 156.8/75.2 - 164.9/67.0

DIPA AA GA GG MA MG

-289.7635720 - 289.7691016 - 289.7753954 - 289.7708762 - 289.7742309

7.42 3.95 0.00 2.84 0.73

0.796 1.047 0.973 0.726 0.870

0.000260 0.000885 0.000873 0.000814 0.000708

HS-C-N-C 178.2 43.3 43.9 -46.7 - 20.5

H-C3-C-N

HlG-C-N-C - 178.0 - 161.6 46.6 162.5 48.8

180.0 58.4 - 179.2 57.0 -56.7

TABLE 2 Geometrical features of IBA related to anomeric interactions* Parameter

Conformer AA

AG

MA

MG

MM

Bond C3-C2 H9-C2 HlO-C2

1.549 1.083 1.083

1.547 1.084 1.083

1.543 1.083 1.089

1.540 1.090 1.083

1.540 1.090 1.083

Angle C3-C2-Nl H9-C2-Nl HlO-C2-Nl

116.3 107.6 107.6

115.8 107.8 107.7

111.5 107.4 112.6

111.3 113.0 107.4

110.6 112.8 107.8

“Bond lengths in Iingstr6ms and angles in degrees.

with the non-planarity hN of the environment of the N atom. When the lone pair is anti to the C2-C3 bond, the N atom is more planar but remains essentially tetrahedral, in agreement with experiment [ 11. Table 4 lists data for the most stable conformers of methylamine [lo], ethylamine [ 111, propylamine [91 and IBA (this work). A comparison of our results with the 6-31G*//6-31G* data for propylamine shows discrepancies for

TABLE 3 Main bond lenw attheNatom (h,) Parameter

and bond anglesb of IBA other than those in Table 2, and the non-planarity

Conformer AA

AG

MA

MG

MM

Bond C2-N1 H4-C3 c5-c3 C6-C3 H7-Nl HS-Nl

1.472 1.086 1.541 1.541 1.000 1.000

1.473 1.086 1.541 1.540 1.001 1.000

1.476 1.086 1.542 1.538 1.000 1.000

1.476 1.083 1.541 1.541 1.001 1.000

1.476 1.086 1.538 1.541 1.001 1.002

Angle H4-C3-C2 C5-C3-C2 C5-C3-H4 C6-C3-C2 C6-C3-H4 C6-C3-C5 H7-Nl-C2 H8-Nl-C2

106.7 111.4 108.2 111.4 108.2 110.7 114.4 114.4

107.9 109.9 108.5 111.4 108.4 110.7 113.8 114.3

106.9 111.3 108.1 110.6 109.0 110.8 113.8 113.8

106.9 110.0 109.3 111.5 108.5 110.6 113.5 113.8

107.7 110.7 108.5 110.2 108.3 111.2 113.8 113.2

hN

0.289

0.297

0.305

0.309

0.309

“In &rgstriims. bin degrees.

TABLE 4 Main geometrical features of the most stable conformers of the primary amines methylamine (MA) [lO],ethylamine (EA) [ll],propylamine (PA) [9] andIBA (thiswork)” Feature Bond N-H C-N c-c

Angle N-C-C C-N-H

MA M.W.

EA 4-31G (N*)

PA 6-31G*

IBA 4-21G

1.093(6)

1.003 1.002 1.462 1.524

1.002 1.002 1.454 1.531 1.528

1.061 1.002 1.476 1.540 1.538 1.541

110.5 110.3 110.4

115.7 110.7 110.7

110.6 113.8 113.2

1.474(5)

112.1(8)

‘Bond lengths in &ngstrgmsand angles in degrees.

all the comparablefeaturesexcept the N-H distances (featureslike C-C-C or C-C-H anglesare omitted becausethese parts of the moleculescan be expected to differ anyway). The 4-21G results are closer to the 6-31G* ones when Schafer’s empirical corrections [ 121 are applied (for C-N, for example, rg-re= - 0.012 A, and for C-N-H, r, - r, = - 4.2” ). Diisopropylumine (DIPA)

The six startingconformations yielded five conformers (Table 1) , since the staggeredconformer GM led to GA. The GG and MG conformers are the most stable,differingby only 0.73 kcal mol-‘. Molecularmechanicscalculations [ 51 have identified GG as the most stable conformer,but portrayedthe next most stable as differing from it by more than 2.5 kcal mol-l. The GED study [5] also identifiedthe GG conformer as the most stable,but with CNCH anglesof 52(4) ’ as against 43.9” and 46.6” calculatedin the present work. The 4-21G study shows that gaucheor -gauche torsional anglesare much narrowerthan 5 60” in order to minimizesteric repulsions.The AA conformer is much more unstablethan the others owing to strong steric repulsionbetweenthe two isopropyl groups. The geometrical features related to anomeric interactions are listed in Table 5. As in IBA, bonds are longer when anti to the nitrogen lone pair than TABLE 5 Geometrical features of DIPA related tc anomeric interactionsa Parameter

Conformer AA

GA

GG

MG

MA

C3-C2 C4-C2 H9-C2 C6-C5 c7-c5 H16-C5

1.548 1.540 1.084 1.540 1.548 1.084

1.539 1.540 1.087 1.540 1.544 1.084

1.538 1.538 1.088 1.544 1.537 1.081

1.539 1.544 1.083 1.545 1.538 1.083

1.540 1.546 1.080 1.538 1.547 1.084

Angle C3-C2-Nl C4-C2-Nl H9-C2-Nl C6-C5-Nl C7-C5-Nl H16-C5-Nl

116.1 112.9 103.2 112.9 116.1 103.2

111.4 107.2 111.7 112.2 114.5 104.3

109.7 108.1 111.8 113.3 108.1 107.2

108.8 113.1 107.0 114.5 108.0 106.5

107.2 115.1 107.4 111.1 115.0 104.9

Bond

“Bond lengths in bgstr6ms and angles in degrees.

324 TABLE 6 Main bond lengths” and bond anglesbof DIPA other than those of Table 5, and the non-planarity at the N atoms ( hN) Feature

Conformer AA

GA

GG

MG

MA

Bond C2-N1 C5-Nl H8-Nl

1.481 1.481 1.001

1.482 1.485 1.002

1.479 1.478 1.004

1.476 1.473 1.002

1.474 1.478 1.002

Angle C4-C2-C3 C5-Nl-C2 C7-C5-C6 H8-Nl-C2 H8-Nl-C5

110.6 127.2 110.6 110.2 110.2

109.7 121.8 111.1 110.2 110.0

110.3 118.4 110.9 111.8 110.8

110.5 120.2 110.7 113.2 113.2

110.2 123.5 111.0 112.0 112.1

h

0.265

0.320

0.327

0.274

0.264

“In ?mgstr~ms. bin degrees.

when gauche, and the N-C-X angle (X = C, H) is wider when the C-X bond is anti to the lone pair. For example, C7-C5 is longer and C7-C&N wider in AA, GA and MA, in which Lp-N-C&C7 is about 180”, than in GG and MG. For a given conformer, the C-C bond that is anti to the lone pair is longer than the other (cf. for instance, C7-C5 with C6-C5 in AA) and the corresponding C-C-N angle is wider. The other main geometrical features of DIPA are listed in Table 6. The least stable conformer has the most planar N atom environment, and the most stable conformer has the least planar N atom environment. Table 7 lists geometrical data for the most stable conformers of the secondary amines dimethylamine [ 13 1, diethylamine [ 111 and ethylmethylamine [ 111 together with GED results for DIPA [ 51 and data obtained in this study. The DIPA geometries differ mainly in the C-H distances, by about 0.033 A; the angles for which the discrepancy is greatest are the C-C-C ones. The 4-21G value of the C-N-C angle (118.4 o ) is intermediate between the GED value (120.1’ ) and the molecular mechanics value (116.1 o ) [5]. Correction of the 4-21G results to r,values (by - 0.012 A for C-N, - 0.008 A for C-C and + 0.034 A for C-H) leads to better agreement with the experimental results. It should be borne in mind, of course, that the GED geometry includes many assumptions. The C-N-C angle is wider in DIPA than in the other secondary amines of Table 7 because of steric repulsion between its bulky N-substituents. This also

TABLE 7 Main geometrical features of the most stable conformers of the secondary aminea dimethyiamine (DMA) [13],diethyIamine (DEA) [ll],ethylmethylamine (EMA) [ll] andDIPA” Feature

DMA M.W.

DEA MM2

EMA MM2

DIPA GEDb

Bond N-H N-C

1.019(7) 1.463(5)

c-c

1.016 1.462 1.462 1.535 1.535

1.017 1.460 1.462

1.532(3)

C-H9 C-H16 Angle C-N-H C-N-C N-C-C3 N-C-C4 c-c-c

1.470(4)

1.121(2) 1.121(2)

108.9(3) 112.2(2)

109.6 109.6 112.4 110.7 110.7

109.4 109.6 112.2 110.8

120.1(10) 111.3(7) 108.9 (5) 112.5(10)

4-21G 1.004 1.479 1.478 1.538 1.544 1.538 1.540 1.088 1.081

111.8 110.8 118.4 109.7 108.1 110.3 110.9

“Bond lengths in Zingstrismsand bond angles in degrees. bTaken from ref. 5.

affects the C-N bond length, which is 1.479 A in DIPA vs. about 1.460 A in the smaller compounds. ACKNOWLEDGEMENT

B.F. acknowledges an FPI grant received from the MEC.

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