J. Phys. Chem. 1994,98, 6105-6109
6105
Ab Initio Study of the Isoelectronic Molecules BCN, BNC, and CJ Including Anharmonicity Jan M. L. Martin Departement SBG, Limburgs Universitair Centrum, Universitaire Campus, B-3590 Diepenbeek. Belgium, and Departement of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium
Peter R. Taylor’ San Diego Supercomputer Center, P.O.Box 85608, San Diego. California 92186-9784 Received: January 20, 1994’ The ground states of BNC, BCN, and C3 were studied using the full-valence CASSCF (complete active space SCF) and CCSD(T) (augmented coupled cluster) methods and basis sets of spdfand spdfgquality. Full quartic force fields were obtained. re and stretching fundamentals for C3 are found to be in excellent agreement with experiment; ro and the bending fundamental reveal shortcomings in the bending part of the potential. The BNC ground state ( 1 P ) is only slightly less strongly bound than C3 and is 9.5 f 0.5 kcal/mol lower in energy than the ground state of BCN. No other low-lying structures or states were found. Stretching fundamentals in BNC are affected by a severe Fermi resonance. Best estimates for the molecular constants are as follows. BNC: ED0 = 300.5 f 1 kcal/mol, r,(CN) = 1.167 A, r,(NC) = 1.416 A, WI = 2075 cm-l, 02 = 998 cm-l, 0 3 = 131 cm-l, ul = 2076 cm-l, u2 = 991 cm-l. BCN: ZDo = 291.0 f 1 kcal/mol, r,(CN) = 1.135 A, r,(BC) = 1.597 A, w1 = 2197 cm-l, 02 = 812 cm-l, 0 3 = 204 cm-l, u1 = 2166 cm-l, Y Z = 810 cm-l. Both stretching fundamentals are computed to have high infrared intensities for both BNC and BCN. Spectroscopic constants for isotopic forms are also given. The best computed ED0 for C3 is 314.7 f 1 kcal/mol, in very good agreement with experiment but with a smaller uncertainty.
Introduction Considerableinterest has been aroused recently by neoceramic materials in the {B,C,N)“magic triangle” of elements. Special cases of mixed {B,C,NJcompounds are the ultrahard materials boron carbide (BC), carbon nitride,’ and 8-boron nitride.2.3 Additionally, boron carbide is of interest in technology because of applications as diverse as nuclear technologye and atomic oxygen-resistant coating of carbon fiber materials in spacecraft.6 The precursors of these materials in surface coating are the BC,N, clusters. Among the heteronuclear clusters, B,N, clusters have been given the most attention, notably for BN,7 B2N,*,9BN2,9JOB3N,I1J2B2N2,13and BN3.12 In the C,N, family, the CN radical is of course very well characterized,14while the C2N and CNZ radicals recently have been the subject of considerablestudy (ref 15 and references therein), as is the case for the cyanogen radical C2N2.16J7 For the B,C, clusters, an in-depth ab initio study is available for BC and B2C18 (see also refs 19 and 20 for BC), as well as a combined ab initio/ experimental study of BCZ.~’ Among the triatomics, this leaves only the ternarysystem BCN undescribed, except for an old HartreeFock-level study by Thomson.22 Since BCN is isoelectronic with C3, interesting comparisons should be possible here. The purpose of the present paper is to present an accurate ab initio study of BCN and the isoelectronic C3, including anharmonic force fields. (It will become evident in the course of the paper that the latter are indispensable to experimental interpretation.)
Methods Two electron correlation methods are used in this work. The first is full-valence CASSCF (complete active space SCFZ3),in which the variationally optimized wave function is a linear combination of all configuration state functions (CSFs, Le. symmetry- and spin-adapted linear combinations of Slater determinants) that can be generated by arranging 12 valence electrons in 12 valence orbitals. This method corresponds to an
* Abstract published in Advance ACS Abstracts. May 15, 1994. 0022-365419412098-6105$04.50/0
exact treatment (within thegivenone-particlebasisset) of internal correlationbut includes no external correlationat all. The second method is the CCSD(T) which is the coupled cluster method25 with all single and double excitations (CCSD)13.26 augmented by a quasiperturbativeestimatefor theeffect of triple excitation^.^^ For systems dominated by a single reference configuration, this method has been shown to yield correlation energies very close to the full configuration interaction (FCI) limit .2733 Initially, SCF-level geometry optimizations were carried out for a number of different structures of BCN using the standard Huzinaga-Dunning DZP (double-{ plus polarization) basis set,29.30 which is a [4s2pld] contraction of a (9sSpld) primitive set. At the optimum geometries, single-point CCSD(T) calculations were then performed. From these results, it was evident that no structures/states other than BNC(lZ+) and BCN(lZ+) needed to be considered. For these and for C3, the following calculations were then carried out. First the geometry was optimized and harmonic frequencies and double-harmonic infrared intensities obtained at the full-valence CASSCF level. Since higher angular momentum polarization functions are intended to account mainly for external correlation effects, Dunning’s correlationansi~tent3~ valence doublaf plus polarization(~~-pVDZ) set,whichisa [3s2pld] contractionofa (8s4pld) primitive set, should beadquate for these CASSCFstudies. (There appears to be an indication that performance of the cc-pVDZ basis set in correlated calculations is generally superior to that of the DZP basis set, especially for harmonic frequencies.32) Optimizationsand harmonic frequency calculationswere carried out using analytical second derivatives. Second, a quartic force field was calculated at the CCSD(T) level using Dunning’s cc-pVTZ (correlation-consistent valence triple-f plus polarization) basis set,31 which is a [4s3p2dlfl contraction of a (10sSp2dlf) primitive set. Pure spherical harmonics were used and 1s-like core orbitals constrained to be doubly occupied. A grid was generated of all points required to generate all nonvanishing quadratic, cubic, and quartic force constants in symmetry-adapted internal coordinates. This 0 1994 American Chemical Society
6106
The Journal of Physical Chemistry, Vol. 98, No. 24, I994
Martin and Taylor
TABLE 1: Computed Total Energies (hartree), Relative Energies (kcal/mol), Geometries (A), Harmonic Frequencies (cm-I), and Infrared Intensities (km/mol) for Ch BNC, and BCN at Different Levels of Theory AE CCSD(T)/DZP geometry (UHF/DZP) frequencies (UHF/DZP) B N C 1Z+ 0.00 -117.26452 rCN = 1.1738,’~~ = 1.4184 2304(~,917),1079(~,246),248(r,9X2) 10.09 -1 17.24844 rCN = 1.1419,rw = 1.5989 2533(~,134),844(~,253),290(r,28X2) B C N IZ+ 77.37 98.57 37.89 44.09 40.15
-117.141 21 -117.107 44 -117.204 13 -117.194 25 -1 17.200 54
IZ+ IZ+ ‘E+,
0 7.12
-117.095 18 -1 17.08384
IZ+ IZ+ ‘E+,
0
-1 17.38602 -1 17.37279
BNC
IZ+
BCN
lZ+
0 9.29
C3
’E+*
-117.360 81 -1 17.34601 -1 13.83297
BNC BCN
IZ+ IZ+
0 9.51 -1 13.86054
-1 17.39029 -117.375 13
NBC NBC BCN BNC BCN BNC
BCN
c3
BNC
BCN c3
c3
311 311 311 3A’
8.30
1.2517,!‘BC 1.4635 = 1.2998,Qc = 1.4672 1.1584,QC 1.4835 = 1.1889,ON 1.3633 = 1.2456,rBN = 1.3211,OCNB = 69.90’ CASSCF[~~,~~]/CC-PVDZ fBN = 1.4570,fCN = 1.2055 Q C = 1.5916,rCN = 1.1809 rcc = 1.3213 CCSD(T)/TZZP = 1.4275,rCN 1.1894 rBC = 1.5815,rCN = 1.1669 rcc = 1.3001 CCSD(T)/W-PVTA‘ QN = 1.4307,rCN 1.1932 Qc 1.5821,r c ~ 1.1702 rcc = 1.3021 CCSD(T)/cc-pVQZd fBN fBN rCN rCN rCN
2167(~,552),1053(~,148),97(a,21X2)
1852(u,24),991(u,< 1),439(ry,17),4O9(rx,56) 2004(u,46),1036(u,38),434(ay,1),343(?r,,2) 1856(u,4),1181 (u,5),435(ry
