Structures and relative stabilities of [C2H6N]+ ions: A non-empirical and MNDO study

Journal of Molecular Structure (Theochem), 124 (1985) 319-324 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

STRUCTURES AND RELATIVE STABILITIES OF [C2H6N] + IONS: A NON-EMPIRICAL AND MNDO STUDY

VINCENZO BARONE* Dipartimento Waly) PASQUALE Dipartimento (Italy)

and FRANCESCO

de Chimca,

Universit6

LELJ

di Napoli,

Via Mezzocannone

4, I-80134

Napoli

GRANDE and NINO RUSSO di Chimica,

Universitci

della Calabria,

I-87030

Arcavacata

di Rende

(Cs)

(Received 11 March 1985)

ABSTRACT Non-empirical molecular orbitalcalculations have been performed on the seven possible isomers at different levels of sophisitcation. The results suggest that the ions possessing a resonance contribution with a filled octet are the most stable ones. Barriers to CH, - and NH, -rotation have also been computed and are generally quite low (_ 5 kJ mol-’ ).

[G&N]+

INTRODUCTION

The [C,H,N]’ ions (m/z 44) provide a prominent peak in the mass spectrum of several amines and mainly decay through loss of acetylene and molecular hydrogen, other reactions (e.g., loss of hydrogen radical or methane) giving only negligible contributions [l-3]. From an experimental point of view, a major problem in constructing reliable potential energy surfaces for these reactions is the lack of heats of formation (AEQ for some reactants and postulated intermediates. In fact, metastable ions dissociation patterns [l] and collisional activation (CA) [ 21 techniques have shown that only the two isomers [CH3CH=NH2 1’ (1) and [CH,NH=CH2]’ (2) are non-interconverting under mass spectrometric conditions. Appearance energies in the ionic dissociations have then provided reliable AH, values of 657 and 695 kJ mol-’ for (1) and (2), respectively. These two ions are formally immonium ions and should, therefore, be more stable than the two isomers [CHz = CHNHB]+ (3) and [CH2NH2C!H2]+ (4). In turn, (3) and (4) should be considerably more stable than the other possible isomers [CH2CH2NH2 1’ (5), [CH3NCH3]+ (6), and [CH3CH2NH]+ (7), since only (l)-(4) can possess resonance structures with completely filled octets [4]. The AH, of (4) is well established (on the basis of the proton affinity of azyridine [5] ) as 716 kJ mol-‘, but the formation enthalpies of (3), (5), (6), and (7) could only be derived from estimation techniques [6] and are, therefore, much less 0166-1280/85/$03.30

0 1985 Elsevier Science Publishers B.V.

320

reliable. The same remarks apply to the possible products of 1,2-hydrogen elimination from (1) (i.e., [CH,-C-NH]+ (8) and [CH2=C=NHZ]+ (9)) and (2) (i.e., [CH2=N=CHJ (10) and [CH,-N=CH]’ (ll)), especially in view of some experimental evidence that Hz loss does not involve the breaking of a bond to nitrogen (i.e., only (9) should be obtained from (1)and (2) should not be able to form neither (10)or (11)by direct loss of H, ) [6] . Furthermore structural data are, of course, not available for any of these ions. A potentially powerful source of structural and energetic information is offered by ab initio quantum-mechanical calculations, provided that the adopted basis set has sufficient flexibility. Recent studies on similar systems [ 7-101 have shown that optimization of geometries employing split-valence basis sets (e.g., 4-31G [ll]) followed by single point computations with basis sets including polarization functions (e.g., 6-31G* [12, 131) provide a very good compromise between economy and accuracy. We have, therefore, followed this procedure for locating all the energy minima. In view of the planned extension of this study to the whole potential energy surface and to larger systems, the same computations have been performed also by the semi-empirical MNDO method [14], which usually predicts geometries and heats of formation of ions in good agreement with experimental and/or sophisticated ab initio computations [ 15, 161. METHOD OF COMPUTATION

Energy minima of the CZH6N* potential energy surface have been searched by gradient techniques, using the 4-31G basis set with analytical slope as implemented in the GAUSSIAN/80 system of programs [17]. In order to obtain more reliable isomerization and elimination energies, single 6-31G* computations have been performed using the 4-31G geometries and these results are denoted by the label 6-31G*//4-31G. MNDO computations have been performed by the MOPAC-II set of programs [18] using the standard parametrization and the DFP optimization technique. RESULTS AND DISCUSSION

The geometries of the various [C2H6N]+ isomers optimized at the 4-31G level are reported in Figs. 1 and 2 and a comparison with STO-3G partially optimized structures [4] shows that the general trends obtained by the two basis sets and the values of the valence angles are very similar. On the other hand, CC and CN bond lengths obtained at the 4-31G level are usually shorter by about 0.2,0.3 A. The relative stabilities of different isomers obtained at various levels of theory are compared in Table 1 with the available experimental data. The experimental stability order is reproduced by all the theoretical methods, except STO-3G, which inverts the order of the pairs (3), (4) and (5), (6). The first inversion is not surprising since it is well known that the STO-3G basis set unduly favours cyclic isomers over acyclic ones [ 19, 201, whereas no clear motivations can be found for the second

321

Fig. 1. Optimized geometries of the ions (l), (2) and (8)-(10) at the 4-31G and MNDO (in parentheses) levels. Bond lengths are in A and valence angles in degrees.

inversion. The very large difference between STO-3G and 4-31G geometries (1.488 vs. 1.418 for the CN bond lengths) cannot, in fact, be responsible for this inversion since 4-31G computations at STO-3G geometries restore the correct stability order. According to all the methods employed there is a clear demarcation of the isomers in two groups, corresponding to ions which can possess resonance structures with completely filled octets (l)-(4) and to ions which cannot (5~(7). Within the first group, the isomers (1) and (2), which are formally immonium ions, are more stable than the other two isomers (3) and (4), and the ion with the terminal NH? group (1) is more stable than the ion with internal NH. This latter result is probably connected with a better stabilization of positive charge on the tertiary carbon atom of (1) than on the secondary carbon atom of (2). From a quantitative point of view, the relative stabilities of (1) and (2) obtained at all the levels of theory are in remarkable agreement with the most recent experimental value of 38 kJ mol-1 [3], and the same applies to the energy difference between the

322

Fig. 2. Optimized geometriesof the ions (4)-(7) at the 4-31G and MNDO (in parentheses) levels. Bond lengths are in A and valence angles in degrees. For the ion (7) structure a is more stable than structure b by 4.9 and 2.9 kJ mol-l at the 4-31G and 6-31G*//4-31G levels, respectively.

N-protonated (3) and C-protonated (1) tautomeric enamines, although in this case the older experimental estimate of 42 kJ mol-’ [6] seems slightly underestimated. Finally, as in the case of [C, H, O]* [ 71 and [C, H501’ [8] isomers, the cyclic N-protonated aziridine (4) is among the more stable structures, notwithstanding its inherent strain. Coming to the second group of isomers, the relative stability of (5) obtained by the ab initio computations is in semiquantitative agreement with the experimental value, whereas the MNDO method overestimates the stability of this isomer. On the other hand, the computed stabilities of (6) and (7) are much lower than the experimental data (which are the less reliable within the series [2, 3, 61) thus suggesting a revision of the experimental values. As far as the conformational preferences and torsional barriers of CHJ and NH3 groups are concerned, these groups usually eclipse double bonds

323 TABLE 1 Relative obtained For the for CH,

stabilities (in kJ mol-’ ) of isomeric [C, H, N]+ ions and fragments [C,H,N]+ by H, elimination from (1) and (2) (see text) ions (l), (2), and (3) the values in brackets are the energy barriers (in kJ mol-‘) or NH, rotations

Ion

MNDO

(l)b (2)

402

STO-3Ga 0 (4.2) 34 (4.4)

4-31G//STO-3G*

4-31G

6-31G*//4-31G

Exp.

3:

0 (4.7) 51 (4.9)

0 (4.9) 40 (5.5)

0 38

I:; (5) (6)

80 5o 220 354

51 72(5.9) 371 326

123 72 430

130 64 (5.4) 309 424

102 79 (5.1) 294 393

42 59 330 205

I:; (9)c (10) (11)

402 400 21 21

413 1 -

500 1 -

4900 45 65 52

4570 67 58 39

190 -

aFrom ref. 4. bThe computed total energy (in a.u.) of isomer (1) is -131.84277 (STO-3G); -133.24801 (4-31G) and -133.44216 (6-31G*//4-31G). 4-31G//STO-3G; -133.25300 ‘The computed total energy (in au.) of isomer (8) is-132.04452 (4-31G) and -132.23639 6-31G*//4-31G.

[19] (see Figs 1 and 2) and the barriers of rotation are very low (“5 kJ mol-’ ) and insensitive to the basis set used (see Table 1). Contrary to the results of ref. 4, we find that this is also the case for both methyl groups of (6) (see Fig. 2). As a final point we have considered the problem of molecular hydrogen elimination from (1) and (2). The results reported in Table 1 indicate that, at all levels of theory, the most stable product is (8), whose production involves the breaking of a bond to nitrogen. The relative stabilities of the other isomers depend on the method used and the most reliable estimate (6-31G*//4-31G) makes (11)more stable than (lo), which, in turn, is more stable than (9). Thus we should conclude that, contrary to the indications of ref. 6, the breaking of a bond to nitrogen is actually the favoured process. However, the transition states for the various processes could be in a different order, thus restoring the agreement with experimental findings, from a kinetic point of view. Work is in progress to analyze this point better together with the mechanism and energetics of the inter-conversion between the different ions through hydrogen shifts. REFERENCES 1 N. Uccella, I. Howe and D. H. Williams, J. Chem. Sot. B, (1971) 1933. 2 K. Levsen and F. W. McLafferty, J. Am. Chem. Sot., 96, (1974) 139. 3 F. P. Lossing, Y. T. Lama nd A. Maccol, Can. J. Chem., 59 (1981) 2228. 4 F. Jordan, J. Phys. Chem., 80 (1976) 76.

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