INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY SYMP. NO. 10, 85-90 (1976)

The Electronic Structure of Unstable Intermediates. VI. The Electronic Structure of AlCN, AlNC, PCN,

and PNC and Comparison with Their 1st-Row Analogues

COLIN THOMSON

Department of Chemistry, University of St. Andrews, St . Andrews, KYl6 9ST, Scotland

Abstract

The electronic structure and bond lengths in AICN, AINC, PCN, and P N C have been investigated using near Hartree-Fock wave functions. A detailed analysis of the Mulliken population analysis and comparison of the results with several other XCN and X N C molecules is presented.

1. Introduction

This paper presents the results of a series of ab-initio LCAO-MO-SCF calculations on the monocyanide and monoisocyanides of aluminium (AlCN and AlNC) and of phos- phorus (PCN and PNC). The first row counterparts of these molecules have been in- vestigated previously [ 11, and we compare the structure and bonding in these compounds in the present paper.

A triatomic species containing Al, N, and C was detected mass spectroscopically some years ago, but no structural parameters are known [2]. The PCN molecule has also been observed spectroscopically [3], but PNC has not yet been detected. We have therefore attempted to provide the experimentalist with some theoretical information regarding the structure and stability of these molecules.

Our early work on similar species BCN and BNC [ 11, and investigations by Clementi and coworkers [4, 53 on LiCN and LiNC, have shown that the equilibrium geometry of this type of molecule can be reliably computed with near Hartree-Fock wave functions, but there has been relatively little previous work on linear intermediates containing second-row atoms [6, 71. We have studied the ground states in this paper and have computed the equilibrium bond lengths in these linear molecules, together with a variety of molecular properties. A subsequent series of papers deals with the potential energy surface for the isomerization reactions XCN - XNC, where X = B or A1 [8].

2. Method of Calculation and Basis Sets

The SCF calculations were carried out with the linear-molecule programme ALCHEMY [9], as described in earlier papers in this series [ l , 101. In view of the large number of electrons (26 or 28) we decided to initially investigate the geometry with a DZ + P basis set, which results in an SCF wave function with an energy substantially above the Har-

85 0 1976 by John Wiley & Sons, Inc.

86 THOMSON

'

Mole cu 1 e R(A1-C) R(C-N) R(A1-N) R(N-C) mergy

-334.25998

- - -334.21888 AlCN 3.827 2.145

AlNC 3.415 2.187 - I -

tree-Fock energy. Our experience in earlier work suggests that the computed bond lengths should still be accurate to within -0.05 bohr. Finally we have computed a BA + P wave function at this geometry and a variety of molecular properties. The polarization function exponents were taken from previous calculations [ 11, or from calculations by McLean and Yoshimine on AIF, FCN, and PN [ 111. These were not reoptimized. Most of the calculations were carried out on an IBM 370/195. All results are tabulated in atomic units.

3. Energy Results: AlNC and AlCN

The molecules AlCN and AlNC are expected to have the same valence electron con- figuration as BCN and BNC, but the possibility of an alternative configuration was also investigated. The two configurations are

C+ (core)(5a)2(6a)2(7a)2(8a)2(2a)4(9a)2 (1)

'C+ ( c o r e ) ( 5 ~ ) ~ ( 6 a ) ~ ( 2 ? r ) ~ ( 7 ~ ) ~ ( 3 ? r ) ~ (2) where (core) denotes the core orbitals.

Calculations showed that configuration (2) was in fact higher in energy by -1 hartree, and all subsequent calculations refer to configuration (1).

Calculations were carried out for various values of R(AlC), R(AlN), R(CN), and R(NC), and the final optimized bond lengths and energies are given in Table I. Both species are predicted to be bound, with SCF atomization energies of 0.302 hartree (AlCN) and 0.321 hartree (AlNC). Hence as was found in the case of the boron compounds [I], the isocyanide is predicted to be more stable. The energy difference of -1 1 kcal/mole is similar to that found in the case of HCN and HNC, except that in this case the HCN molecule is more stable [ 121. However, this difference is only about half that found be- tween the BCN and BNC molecules, and it seems likely that the question of the relative stabilities is less easily settled in this case without extensive CI. It should be noted, however, that the inclusion of CI in the HCN - HNC calculations made very little difference to the relative stabilities of the isomers (only -5 kcal/mole extra stabilization of the HNC molecule) [ 121.

There are no simple compounds for which the Al-C or Al-N bond lengths are known, but the values are similar to observed Al-C and Al-N bond lengths in [(CH3)2Al-C1]2 [13] and (CH3AlNCH3)7 [14]. The bond lengths R(CN) and R(NC) are rather similar, although in all these compounds R(NC) is ~ 0 . 0 5 bohr longer than R(CN).

Table I1 lists the valence shell orbital energies at the computed equilibrium geometry. Differences between the DZ + P and BA + P calculations are slight. The energies are very similar for both AlCN and AlNC. In AlNC the 2?r orbital is lower in energy than the 8a orbital, the reverse of the situation in AlCN. The order of the energy levels in both BCN and BNC is as in AlNC. In LiCN the highest occupied MO is the ?r orbital [4,5], whereas in LiNC it is the 6a orbital. Thus LiCN and AlCN are similar in this respect

ELECTRONIC STRUCTURE OF UNSTABLE INTERMEDIATES 87

I DZ + P basis Orbi ta l A l C N A l N C

6 d -1 .2121 -1.2413

7d -0.6665 -0.7513

-0.5454 -0 .4772

-0 .4700 -0 .5062

-0.3418 -0.3417

EL4 + P b a s i s A l C N A l N C

-1 .2150 -1 .2383

-0 .6690 - 0.7485

-0 .5469 -0.4751

-0 .4723 -0 .5033

-0 .3430 -0 .3402

TABLE 111. Optimized bond lengths and energies for %-PCN and PNC (DZ + P basis).

(Molecule 1 R(P-C) I R(C-N) I R(P-N) I R(N-C) I mergy I -433.0509 I 2:14 1 3.18 I 2 . i 8 I -433.0340 I

and different from BCN, but these are very small energy differences which may well disappear with a near Hartree-Fock function, or if correlation is included.

The experimental information available on the aluminium species is restricted to an estimate of the atomization energy of 300 kcal/mole [2]. Our computed values for AlCN (190 kcal/mole) and AlNC (201 kcal/mole) are expected to be lower than the true value [ 151. The appearance potential measured in [2] was 7.4 f 0.3 eV. The first ionization potential, assuming Koopman’s theorem, is calculated to be 0.34 hartree (9.2 eV) for both species, which is not in very good agreement with experiment, but is practically the same for both DZ + P and BA + P basis sets.

4. Energy Results: PNC and PCN The ground-state configuration of these molecules was assumed to be similar to that

of NNC, giving the configuration

3C- (core)(5~)~(6~)~(7~)~(8~)~(9~)~(21r)~(31r)~ We have only used the DZ + P basis set for these molecules at the present time in view of the cost of the computations. The optimized geometries and energies are given in Table 111, and the valence orbital energies in Table IV.

In this case PCN is predicted to be more stable. In the light of the above results for the boron [ 11 and aluminium compounds it seems unlikely that this conclusion would be changed with a larger basis set. It is also consistent with the experimental results which established the presence of PCN following flash photolysis of a mixture of PH3, C2N2, and N2 [3]. The authors observed the 311 - 3C- electronic transition in this work. However, since in the case of the nitrogen analogues NCN is the more stable [ 11 and NNC has only been detected relatively recently in matrix isolation experiments [ 161, it seems likely that both PCN and NCN are more stable than the corresponding isocy- anides. The computed atomization energies at this level of approximation are 0.276 hartree (PCN) and 0.259 hartree (PNC).

88 THOMSON

TABLE IV. Valence orbital energies for the computed equilibrium geometries of PNC and PCN.

L

Li +0.5

B +0.2

C -0

N -0.1

0 -0.2

F -0.1

A1 +0.6

P +O. 5

1 - Orbital

b +9.2

+0.5

+1.18

0

-0.52

+0.6c

+3.4

+3.16

8 6

9&

2x

3%

+0.8

+0.1

-0.1

Ico

- -

+0.5

+0.5

PCN

-1.247

-0.885

-0.605

-0.533

-0.512

-0.410

DZ + P b a s i s

-1.286

-0.992

-0.590

-0.487

-0.554

-0.385

TABLE V. Computed charges and dipole moments in XCN and X-NC calculated for DZ + P basis sets.

XCN

-0.3

-0

+0.1

+o. 1

+0.3

+o. 2

-0.4

-0.3

% - -0.2

-0.2

-0.1

-0.1

-0.1

-0.1

-0.2

-0.2

-

-

XNC

qN qC

+o. 1 -0.1

+0.1 -0.1

-0.8 I +0.3 -Oa9 I +Oe4

Y

+8.3

-0.0

0

+1.02

- -

+2.6

+2.56

6 + 6 - 6 + 6 - aValues in debyes. A positive value implies polarity XCN or X-NC. b From [ 31 . Gaussian basis. CFrom [ 181.

5. Population Analysis of X-CN and X-NC Calculations at this level of accuracy have now been completed for the following cy-

anides and isocyanides: HCN and HNC [ 121, LiCN and LiNC [4,5], BCN and BNC [ 13, CCN and CNC [ l ] , NCN and NNC [ 11, OCN [17], and FCN [ 11, 181. Only in the last two cases are calculations on the corresponding isocyanides missing. Since one striking result of these calculations is the approximately constant value for R(CN), especially for the closed-shell molecules where R(CN) = 2.16 f 0.03 bohr, R(NC) = 2.19 f 0.03, analysis of the charge redistribution in this series of molecules X-CN and X-NC for approximately constant R(CN) and R(NC) but different X is of some interest. We have therefore carried out a Mulliken population analysis of the wave functions at the computed geometries, and Table V presents the charges q on the atoms in this ap- proximation for this series of molecules. We have tabulated q for the DZ + P basis sets to facilitate comparisons among these molecules, although there are significant differences between q computed for the same molecules with DZ + P and BA + P basis sets [ l] . Nevertheless, the trends should be meaningful. Also presented in Table V are the com-

ELECTRONIC STRUCTURE OF UNSTABLE INTERMEDIATES 89

puted dipole moments which also reflect the redistribution of charge on molecule for- mation. These are presented for all the molecules, although in the case of LiCN and LiNC the results were obtained using a large Gaussian basis set [4], and for FCN a DZ basis was used [ 181.

The large dipole moments in LiCN and LiNC are similar in magnitude but due to quite different charge distributions. In LiNC the large charge separation is almost all in the Li-N bond and the carbon is almost neutral, whereas in LiCN the CN bond is much less polar. Similar trends are seen in AlCN and AlNC, although the case of AlNC is less extreme and a lower p is computed. On the other hand, both BCN and BNC have rather small dipole moments, and the molecular charge displacements are small. The computed sign of p is negative for BNC. However, in this case there is a large difference between the q computed with DZ + P and BA + P basis sets, and in view of the well-known dif- ficulty of computing small values of p with SCF wave functions, it is likely that the sign is incorrect in this instance. However, CI calculations will be needed to clarify this point. Both PCN and PNC are both predicted to be highly polar, with charge distributions similar to those in the A1 compounds. A more accurate picture of the charge distribution in these molecules is provided by electron density maps, and work is under way to compute them for these molecules.

Returning now to a more detailed discussion of the bonding in the A1 and P compounds, a detailed analysis of the populations by basis function (not presented here) shows that the charge transfer is mainly due to u-electron transfer from A1 to C (and to a lesser extent N) in AlCN, whereas in the AlNC molecule, large u shifts from A1 are reflected in almost zero u charge on N but a large negative a-electron density on C. a-electron shifts are much larger in the isocyanide, particularly for C -+ N. The charge shifts in PCN and PNC are qualitatively rather similar to those in the A1 compounds.

6. Molecular Properties

Although we have computed a large number of expectation values for these molecules, we do not report them in detail in this paper since experimental work is lacking. The dipole moment results were discussed above, and it will be interesting to see if the predicted large dipole moments of these molecules are found. One possible consequence of the large positive charge found in these calculations on A1 or P is the possibility of ready electron capture to form a stable negative ion [ 191. We are currently investigating this possibili- ty.

7. Conclusion

Ab-initio SCF calculations of the quality described here can give useful information on the bonding and charge distribution in triatomic linear molecules containing 2nd-row atoms, and although extensions to molecules such as GaCN are feasible (and currently under way [20]), such calculations are relatively expensive. However, for the type of intermediates dealt with in this paper, theoretical calculations may well provide molecular information at least as cheaply as experiment, and therefore the cost of the computations is more than justified. Further work on this type of species is under way in this laboratory.

Acknowledgment

The author would like to thank Prof. Lowdin and the organisers of the Symposium for the invitation to present his work at Sanibel and for financial assistance. He also wishes

90 THOMSON

to thank the Royal Society for a travel grant and the Science Research Council for a grant of computer time on the IBM 370/195 at the Rutherford Laboratory.

Bibliography

[ I ] C. Thomson, J. Chem. Phys. 58,216 (1973); 841 (1973). [2] K. Gingerich, Naturwiss. 24,646 (1967). [3] N. Basco and K. K. Yee, Chem. Comm. 152 (1968). [4] B. Bak, E. Clementi, and R. N. Kortzeborn, J. Chem. Phys. 52,764 (1970). [5] E. Clernenti, H. Kistenmacher, and H. Popkie, J. Chem. Phys. 58,2460 (1973). [6] C. Thomson, Chem. Phys. Lett. 25,59 (1974). [7] C. Thomson, Theor. Chirn. Acta 35,237 (1974). [8] C. Thomson and R. J. Russell, to be published. [9] A. D. McLean, IBM Tech. Rep. RA 78 (1971).

[ lo ] C. Thomson and B. J. Wishart, Theor. Chim. Acta 31,347 (1933); 35,261,267 (1974). [ I I ] A. D. McLean and M. Yoshimine, IBM J. Res. Develop., Suppl. 12, 1 (1968). [I21 P. K. Pearson, H. F. Shaefer, 111, and U. Wahlgren, J. Chem. Phys. 62,350 (1975). [13] K. Brendhangen, A. Haaland, and D. P. Novak, Acta Chem. Scand. A28,45 (1974). [14] P. B. Hitchcock, G. M. McLaughlin, J. D. Smith, and K. M. Thomas, J.C.S. Chem. Comm. 934 (1973). [ I 51 M. Yoshimine and A. D. McLean, Int. J. Quant. Chem. S1,313 (1967). [ 161. G. R. Smith and W. Weltner, Jr., J. Chem. Phys. 62,4592 (1975), and earlier work referred to in this

[ 171 C. Thomson and B. J. Wishart, Theor. Chim. Acta 35,261 (1974). [I81 M. Dixon, G. Doggett, and G. Howat, J. Chem. Soc., Farad. Trans. II.71,452 (1975). [ 191 J. Simons, private communication. [20] C. Thomson, unpublished calculations.

Received March 26, 1976

paper.