7226
J. Phys. Chem. 1992,96,7226-7230
Experimental and Theoretkail Studies of Photoelectron Spectra of Oxetane and Some of Its ifalogenated Methyl Derivatives Szczepan Rosza4t Joyce J. Kaufman, W. S. Koski,* Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21 218
Reynaldo D. Barreto, T. P. Fehlaer, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 and K. Blrhsubnmanian Department of Chemistry, Arizona State University, Tempe, Arizona 85287- 1604 (Received: February IO, 1992; In Final Form: May 4, 1992) The photoelectron spectra of oxetane, 3-chloromethyl-3-methyloxetane,3-bromomethyl-3-methyloxetane,and 3-iodo-3methyloxetane were studied using He I (21.22 eV) photons. Satisfactory agreement was obtained with ab-initio MRDCI molecular orbital calculations permitting an assignment of the observed bands.
Introduction This work was undertaken in connection with an earlier quantum chemical studylJ of the mechanism of ionic polymerization of oxetane and some of its related compounds in the syntheis of energetic polymers. Since this work has not been reported and since there is an interest in accurate measurements of the ionization potentials of organic compounds, we present our results at this time. In this paper we report our experimental and themtical studies of the photoelectron spectra of oxetane, 3-chloromethyl-3methyloxetane (CIMMO), 3-bromomethyl-3-methyloxetane (BrMMO), and 3-iodomethyl-3-methyloxetane(IMMO). Because of the complicated electronic structure, the interpretation of the photoelectron spectra of oxetane and its halogen derivatives prtsents a challenge to quantum chemistry. Our fmt trials together with the literature data3 show that reliable predictions of the ionization potentials require methods that include correlation energy within at least double-zeta basis set supplemented by a set of d polarization functions. To perform abinitio calculations on 64-electron chloromethyloxetane, 82-electron bromomethyloxetane, and 1Welectron iodomethyloxetane, one has to reduce the size of the problem by making approximations in the method used. The number of electrons has been reduced by applying the effective core potential approximation. For oxetanes involving the heavier halogens where relativistic effects become sisnificant, relativistic effective core potentials (REP) are used. To make calculations feasible, the CH3 group has been replaced by a hydrogen atom. This leads to 110 molecular orbitals in the largest case studied. For the proper interpretation of the band splitting due to spin-orbit effects, additional calculations have been performed on the hydrogen halides. The agreement between computational results and measured spectra has been found satisfactory. Calculations at the MRDCI level predict a proper order of bands, and in most cases the calculated and experimental results differ by less than 1 eV.
treatment is of the standard multiple reference singlet and doublet configuration interaction methods (MRDCI) developed by The computations are carried out employing the Buenker et 91.6~~ Table CI algorithm? The number of symmetry-adapted functions (SAFs) generated by single and double excitations for a given electronic state was reduced by employing an energy selection criterion9 with an assumed threshold T. The eigenvalues of diagonalizing the matrix corresponding to T together with the summed perturbative energy lowerings of the nonselected configurations in the total MRDCI space have then been employed'O to determine an extrapolated zero-threshold value (Ea,MmI). The energy corresponding to the full CI treatment has been estimated via the relation
EFCI= EEX.MRDCI + (EEXMRDCI - E d 1 - CC?) (1) which is a generalization" of the correction formula suggested by Langhoff and Davidson,I2where E,, is the reference secular equation energy. The Davidson formula was shown by Paldus13 to be a good correction for the lack of size extensivity of the energy with single and double excitations. In a recent artide,lm Buenker and co-workers have shown that there is an excellent agreement of the MRDCI method (as described above) with a large series of full CI benchmark calculations. The MRDCI calculationswere performed using integral and SCF routines of the ATMOL 3 program" and transformation and CI routines of the MRDCI program.15Onaclectron integrals within the formalism of relativistic effective core potential were generated by the GAMESS program.16 Relativistic CASSCF/soCr/RCI Computations on HX nod HX+. Since the spin-orbit effects are quite important for both Br and I atoms, we decided to compute the effect of spin-orbit coupling on the ground state of HX and ground and excited electronic states of HX+ (X = C1, Br, I). Since the ionization of halogenated oxetanes takes place on the halogen atom, HX molecules are good models to gain insight into the effect of spin-orbit coupling on the electronic states of the oxetane ions. Note that in HX compounds we consider too the ionization is primarily on the halogen atom. Experimental Section We employed relativistic effective potentiab of Pacios and The photoelectron spectrometer and procedures for sampling, Christan~en,'~ Hurley et a1.,18and LaJohn et aI.l9 for C1, Br, and handling, and calibration are the same as those described in earlier I, respectively. Valence (3s3p2d) Gaussian basis sets were used work's Highly purified samples of the oxetanes used in this study for C1, Br, and I atoms. We start with the (3s3p) basis sets were supplied by G. E.Manzer (Aerojet Solid Propulsion) and reported by these authors optimized for the zPground states of used as received. these atoms. To this basis set, two sets of d polarization functions "beoretical Methodology were added. For the hydrogen atom we used the (4slp) van basis set.20 M u l t i p k R ~ ~ s i p o k a n d ~ E x ~ ~ c o n f i e a t i Dujineveldt on We start with the complete active space MCSCF method Interaction Metbod (MRDCI).The configuration interaction (CASSCF) to generate the orbitals for CI calculations. The CASSCF method included the valence s and p orbitals of the 'Permanent address: Institute of Organic and Physical Chemistry, Wyb. halogen atom and the 1s orbital of the hydrogen atom. Eight Wyspianskiego 27, PL-50-370(1-4), Wroclaw, Poland. 0022-36S4/92/2096-7226$03.00/0
Q 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7227
Photoelectron Spectra of Oxetane
i
I
.H
I
f
11
Ill
IV
v
Figure 1. Geometry of halomethyloxetane.
valence electrons were distributed in all poasible ways among these orbitals at the CASSCF stage. Following the CASSCF method, we use the second-order CI (SOCI) method to include higher-order electron correlation effects and to generate the natural orbitals which are in turn used in the relativistic CI. The SOCI calculations included (i) all configurations in the CASSCF, (ii) configurationsgenerated by distributing seven electrons in the internal space and one electron in the external space in all possible ways, and (iii) conf@rations generated by distributing six electrons in the internal space and two electrons in the external space in all p i b l e ways. Separate CASSCF/SOCI calculations were done for each state of the neutral molecule and ion. All calculations were done in the C, symmetry for convenience. The spin-orbit coupling was included using the RCI method. Spin-orbit integrals were obtained using the differences of 1 '/*and 1 - RECPs and included in the CI matrix. The SOCI natural orbitals were used for the RCI. This procedure is described in refs 21 and 22. The RCI calculations of the O+ ground state included five reference configurations arising from l2? and 3&(ur3) states. The state of HX+ included eight reference configurations arising from 2nl2, zZ:/2, 42;/2,2Z;/212and 2TI2 states. Likewise, the RCI calcuiations of the 3/2 state included seven reference configurations arising from 2113/2, 42;,2, and 'A312 states. Single plus double excitations were allowed from all these reference configurations at the RCI stage. All CASSCF/SOCI calculations were made using one of the authors' (K.Balasubramanian) modified version of ALCHEMY I1 codes to include relativistic ECPs. The spin-orbit integrals over Gaussian basis sets were evaluated using R. Pitzer's ARGOS codes. The RCI was done using the method in ref 21.
1 10.0
12.0
14.0
16.0
18.0
IonizationPotential(ev)
Figure 2. He I spectrum for oxetane.
+
I
computationrl Details b m e & y Oprimiptioa A full geometry optimization has been
performed for the ground state of oxetane and the halogenated methyl derivatives assuming the symmetry plane through OC3C4X atoms (Figure1). The optimization has been performed at the SCF level, using the GAMESS program.I6 The Huzinaga/Dunning double-zeta basis set24has been employed for the oxetane optimization. The geometry optimizations for the halomethyloxetanes have been performed employing the formalism of the relativistic effective core potential (REP).17 The minimal basis sets including REP were used for C, 0, Cl,I7 Br,18 and 1,19 and the van Dujineveldt20minimal (4s) basis was used for the hydrogen atom. MRDCIcalcdatLulp. Oxetune Molecule. MRDCI calculations have been performed for the ground state of the oxetane molecule and for a number of excited states of oxetane cation in the geometry of the neutral molecule. The Huzinaga/Dunning basis set2' has been employed. The basis consists nine s and five p functions in a [3s2p] contraction for carbon and oxygen atoms and four s functions contracted to [ a ] for hydrogen. In addition, a set of Cartesian d polarization functions has been placed at each of the carbon and oxygen atoms with exponents taken from the work of Huzinaga et al.25and one p polarization function with the exponent of 1.O on the hydrogen atom. This choice leads to a total of 90 atomic orbitals. A core of four molecular orbitals corresponding to the 1s shells of oxygen and three carbon atoms
10 0
120
140
160
180
ionizationPotential (ev)
Figure 3. He I spectrum for 3-chloromethyl-3-methyloxetane.
has been kept doubly oocupied in all calculations, and the 26 virtual MOs with the highest orbital energies have been excluded. The remaining 60 MOs are used for generation of single and double excitations relative to a set of nine reference codigurations, about 1 500000 symmetry-adapted functions (SAFs) in all. By applying the selection threshold of 30 crhartrees, approximately 9000 SAFs were included in diagonalization. The estimate of the full CI energy has been performed by applying the extrapolation procedure and adding the Davidson correction. Halomethyloxetanes. MRDCI calculations have been performed for a set of three halomethyloxetane molecules including chlorine, bromine, and iodine. The number of electrons has been reduced by u t i l i i g relativistic effective core potentials (REP'S)" for all atoms. The s2pXvalence configuration was included for C, 0,C1, and Br atoms, while the d'Os2p5valence shell was used for I. The Gaussian atomic basis sets for C, 0, and C1 have been adopted from the work of Pacios and Christian~en.'~In each casc the highest exponent has been left uncontracted. C and 0 atoms were supported additionally by one d-polarization function pro-
7228 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 TABLE I: Symmetry and Mulliken Atomic Population for Mokular Orbitals Resulting from SCF Cal~ulati~ns for the Oxetane Molecule MO MO no. symmetry Mulliken atomic population 16 a’ 0 (1.18), H (0.13) 15 a’ 0 (1.06), C2 (0.19),C3 (0.40) 14 a” C2 (0.48),C3 (0.60) 13 a’ 0 (0.40). C3 (0.77) 12 a” 0 (0.86),C2 (0.41),C3 (0.22) 11 a’ C2 (0.55) TABLE 11: Symmetry and Mulliken Atomic Population for Molecular Orbitals Resulting from SCF Calculations for the Oxetane Cation MO MO no. symmetry Mulliken atomic population 16 a’ 0 (0.93) 15 a” C2 (0.54),C3 (0.62) 14 a’ C2 (0.16),C3 (0.80) 13 a’ 0 (0.72),C2 (0.21),C3 (0.61) 12 a” C2 (0.63) 11 a” 0 (l.Ol), C2 (0.34),C3 (0.14) TABLE III: Vertical Ionization Potentials (eV) from SCF and MRDCI Calculations and Experimental Data for the Oxetane Molecule IP IP symmetry of state SCF MO” MRDCI M o b IP(expt) band A‘ 11.19 16 9.25 16 9.65 I A’ 12.25 15 10.38 13 11.35 I1 12.84 14 11.81 15 12.18 I11 A“ A‘ 14.61 13 12.99 14 13.33 IV 11 14.00 V A” 15.50 12 14.68 “Molecular orbitals from Table I. bMolecular orbitals from Table 11.
p e d by Dunning and Hay?6 and C1 basis set has been extended by two d polarization functions of Huzinaga et alazs The Br minimal basis set was taken from Hurley et a1.I8 and has been extended by two s, two p, and two d functions as proposed by Chapman et ale2’ The basis set for iodine has been taken from LaJohn et al.19 The minimal basis set for dIos2p5valence shell was later extended with one s type and one p type function, and a pair of d-type functions was added to the basis with exponents proposed by Chapman et al.27 The van Dujineveldt20(4s) basis set contracted to [2s] has been employed for all hydrogens. The assumed basis set in the case of chloromethyloxetaneleads to 104 basis functions. After core electrons were removed, the system poseses 36 electrons. Ten highest virtual molecular orbitals were removed at the transformation stage. The bromomethyloxetane is represented by 108 basis functions and has 36 electrons after removing core electrons. Ten highest virtual MOs were discarded. In the iodomethyloxetanethe choice of basis set leads to 110 orbitals. The molecule has 48 electrons after removing core ones. Five occupied orbitals were frozen, and 15 were discarded at the transformation stage. In each case MRDCI calculations have been performed for the ground state of the neutral molecule and for a number of the excited states of the cation. Ten to fourteen main reference configurations are employed. The threshold of 30 phartrees reduced the number of selected configurations to about 7000. Calculations of extrapolated CI energy and a Davidson correction permitted an estimate of the full CI energy.
Resulta .ad Discussion Oxetrae. The results of SCF (Koopmanns’ approximation) and MRDCI calculations are presented in Tables 1-111. The first band of the oxetane photoelectron spectra corresponds to removal of an electron from the HOMO molecular orbital which is mostly localized on the oxygen lone pair. The band is split due to vibrational effects.** Its averaged value of 9.65 eV agrees with the calculated value (MRDCI) of 9.25 eV. The results of calculation
Roszak et al. TABLE I V Symmetry and Multiken Atomic Population for Molecular Orbitals Resulting from SCF Calculations for 3-Cbloromethyloxetane MO MO no.” symmetry Mulliken atomic population 18 a’ 0 (1.02),C2 (0.10), C3 (O.ll), CI (0.38) 17 a” CI (1.68) 16 a’ 0 (0.59),C4 (0.10),CI (0.88) 15 a’ 0 (0.73),C3 (0.23),CI (0.62) 14 a” C2 (0.41),C3 (0.62),CI (0.21) 13 a’ C3 (0.18),C4 (0.45),CI (1.12) 12 a” 0 (0.99),C2 (0.36),C3 (0.11) a
Core orbitals not included.
TABLE V Symmetry and MulliLen Atomic Population for Molecular Orbitsla Resulting from SCF Calculations for tbe 3-Chloromethyloxetane Cation MO MO n0.O symmetry Mulliken atomic population 18 a’ 0 (0.97) 17 a” CI (1.86) 16 a’ C1 (1.84) 15 a’ C3 (0.13),C4 (0.46), CI (1.17) 14 a’ 0 (0.64),C2 (0.15),C3 (0.49),C4 (0.37), CI (0.11) 13 a” C2 (0.47),C3 (0.52),C4 (0.13) 12 a’ 0 (0.21),C2 (0.15),C3 (0.54),C4 (0.34).CI (0.12) Core orbitals not included.
TABLE VI: Vertical Ionization Potentials (eV) from SCF and MRDCI Calculations and Experimental Data for 3-Chloromethyloxetaw 1P symmetry IP of state SCF MO” MRDCI M@ IP(expt) band A‘ 11.44 18 9.68 18 9.76 I A’’ 11.82 17 11.92 17 11.09 I1 A‘ 11.83 16 11.68 14/16 11.33 111 A‘ 12.21 15 12.40 16/14 11.69 IV A” 12.66 14 12.45 13 12.96 V A‘ 13.57 13 13.82 VI
“ Molecular orbitals from Table IV.
Molecular orbitals from Table
V.
for vertical ionization potentials agree well with experimental results (Table 111). Bands I1 and V are related to excitation from delocalized molecular orbitals composed from carbons and oxygen. Bands I11 and IV are related to MOs of oxetane ring excluding oxygen. The vibrational splitting of the oxygen band being 0.15 eV agrees with the splitting of the analogous band of the ethylene oxide spectra (0.16 eV).29 Bands I, 11, and V of the oxetane spectra have corresponding analogues in the spectra of ethylene oxidesz9 “ e t h y l Derivatives of Oxetane. The halogenated methyloxetanes investigated in this study form an interesting series of compounds since increasing the atomic numbers of halogen results in the systematic changes in the photoelectron spectra (Figures 2-5). Atomic Mulliken populations of molecular orbitals are presented for a neutral molecule (useful for the interpretation of SCF results within Koopmanns’ approximation)and for molecular orbitals of the cation (utilized in MRDCI calculations for excited states of the cation). Because of molecular symmetry, the pop ulation on C2Aand CZBatoms is always the same and is presented in the tables as C2. Results of calculations for ClMMO are presented in Tables IV-VI. The first band corresponds to molecular orbital localized on the oxygen lone pair. The second band corresponds to the ionization potential from MO being mostly chlorine lone pairs. This band resembles closely the ionization potential of chlorine in the CHJCl ~ p a c t r a .The ~ ~ BrMMO calculations are presented in Tables VII-IX. The first band corresponds to ionization from oxygen lone pairs, as is properly predicted by the MRDCI method. The second band corresponds to ionization from MO localized
Photoelectron Spectra of Oxetane TABLE VII: Symmetry ud MIllliLcn Atomic Popplrtion for Mdcculrr Orbitals R d t h g from SCF Caleuhtiom for 3-B”ethyloxetane
MO MO no.” symmetry 18 a’ Br (1.72) 17 a’‘ Br (1.87) 16 15
a’
14 13 12
a”
TABLE XI: Symmetry and Mulllken Atomic Population for Molecular Orbitals Resulting from SCF Calculations for the Iodomethyloxetane Cation
MO Mulliken atomic population
n0.O
23 22 21 20 19 18 17 16
0 (1.24), Br (0.14) 0 (0.95), C2 (0.18), C3 (0.28), C4 (0.12), Br
a’
(0.14) C2 (0.45), C3 (0.65) 0 (0.12), C3 (0.16), C4 (0.52), Br (1.18) 0 (0.99), C2 ().36), C3 (0.11)
a’ a” Core orbitals not included.
MO no.”
MO symmetry a’ a’’ a’ a’ a’ a” a’
18 17 16 15 14 13 12
Mulliken atomic population 0 (0.97) Br (1.90) Br (1.91) C3 (OJO), Br (1.17) 0 (0.63), C2 (0.15), C3 (0.51) C2 (0.46), C3 (0.51) 0 (0.21), C2 (0.15), C3 (0.61)
oCoreorbitals not included. TABLE I X Vertical Ionization Potentials (eV) from SCF and MRDCI c.lcULtiom rad Experimental Data for the 3-Bromometbvloxetane IP IP symmetry of state SCF MW MRDCI Mob IP(expt) band I 18 9.54 18 9.68 A’ 10.96 17 12.10 11.01 A” 17 10.28 11, 10.56 IIb A’ 11.62 16 9.64 14/16 10.92 I11 A’ 12.11 15 13.44 16/14 11.39 IV A” 12.65 14 12.34 11 13.20 V A‘ 13.06 13 13.85 VI ~~
Molecular orbitals from Table VII. Molecular orbitals from Table VIII. TABLE X Symmetry rad MulliLen Atomic Population for Molecular orbit& Readting from SCF Calculations for the IoaoWthyloxetane Molecule
MO
MO symmetry a’ a“ a‘
a‘ a‘
a”
a” a’
MO symmetry a’ a” a‘ a’ a’
a” a” a’
Mulliken atomic population I(O.99) I(1.96) 0 (1.41) 0 (0.49), C3 (0.19), C4 ( 0 - l l ) , I(0.87) 0 (0.64), C3 (0.19), C3 (0.22), I(0.73) C2 (0.45), C3 (0.69) 0 (0.96). C2 (0.37), C3 (0.15) 0 (0.26), C2 (0.15), C3 (0.47), C4 (0.55)
Core orbitals not included.
TABLE VIR Symmetry md MulllLea Atomic Population for Mdceulu orbitals Resulting from SCF c.lculrtiom for the 3-B”ethybxttane Cation
no.‘ 23 22 21 20 19 18 17 16
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 1229
Mulliken atomic population I (1.93) I (1.94) 0 (0.94), C3 (0.13), C4 (0.10), I(0.30) 0 (0.81), C4 (0.27), I(0.50) 0 (0.64), C2 (0.12), C3 (0.27), C4 (0.36), I(0.46) C2 (0.46), C3 (0.65) 0 (0.99), C2 (0.36), C3 (0.10) 0 (0.26), C2 (0.10), C3 (0.14), C4 (0.50)
“Core orbitals not included.
on bromine lone pairs. The SCF calculations predict the reverse order of these bands. The shape of the ‘bromine” band resembles the corresponding band in CHpBr.29 Contrary to the results for the previous molecules, the first band in IMMO corresponds to the ionization from the lone pairs of iodine. This fact is properly predicted by both SCF and MRD-CI calculations (Tables X-XII). The first band is again similar to the one corresponding to iodine on the spectra of CHJ molecule.29 The second ionization potential is related to molecular orbitals localized mostly on oxygen lone pairs. As before, higher bands correspond to very delocalized MO’s involving the ring and iodine atom. In general, the observations on the halogenated methyloxetanes are consistent with
TABLE XII: Vertical Ieniution Potentials (eV) from SCF and MRDCI Calculationsand Experimental Data for Iodomethyloxetane Molecule IP IP symmetry of state SCF MOO MRDCI M e IP(expt) band A“ 9.94 22 9.25 22 9.33 I, 9.97 Ib 23 9.67 I1 9.93 23 8.96 A‘ 10.48 19 10.88 I11 A’ 11.55 21 IV 20 11.15 21 11.21 A‘ 11.85 18 12.53 V 12.64 18 12.34 A”
Molecular orbitals from Table X. Molecular orbitals from Table XI.
TABLE XIII: Computed Spectroscopic Constants for HX and HX+ Together with Available Experimental Data re (A) we (cm-I) 1P.d (ev) s&es state theory expt theory expt theory expt 2316 HI 0’ 1.609 1.609 2316 HI+ ’II,/2 1.632 (1.62) 2220 (2170) 9.833 10.4 10.511 11.1 2219 HI+ 2111/2 1.632 13.838 13.852 1083 HI+ 22:/, 1.890 2649 HBr 0’ 1.419 1.414 2628 2442 11.232 11.67 HBr+ ’II,/2 1.453 1.448 2435 11.572 HBr’ 2111/2 1.453 243 1 1404 15.163 15.2 HBr+ 22:/2 1.694 1.684 1338 2990 1.284 1.275 2986 HC1 O+ 2674 12.255 12.75 HCI+ 211,/2 1.326 1.314 2736 HCI+ 2111/2 1.326 12.343 2736 1606 16.04 16.3 HCI+ 22:/2 1.508 1.51 1654
photoelectron data on other halogenated organic molecules.30 The computed ionization potential for the A” state of 3bromomethyloxetane (Table IX) is somewhat less accurate compared to the corresponding iodooxetane (Table XII) because electron correlation effects and basis set size seemed to be more important for bromooxetane while relativistic effects appear to be more important for iodooxetane. In a sense this also appears to be consistent with results in Table XI11 which were obtained at a much higher level of theory so the correlation effects, basis set size, and relativistic effects are more accurately taken into account, and the apparent errors between calculated and experimental ionization are smaller and more consistent.
Results of Spin-Orbit Calculations on H X and HX+ Table XI11 shows our computed CASSCF/SOCI/RCI results including spin-orbit coupling on HX and HX+ (X = I, Br, Cl). As seen from Table XIII, our computed results for HX and M + are in excellent agreement with known experimental results. However, there are several new results especially concerning re and we values of the excited electronic states of HX+. As seen from Table XIII, the 2113,2-%1 computed splittings for HI+, HBr+, and HCP are 0.68,0.34, and 0.09 eV, respectively. For HI+, the corresponding experimental value23is known and it is 0.7 eV, in excellent agreement with the computed splitting. Although the adiabatic IP is underestimated by 0.5-0.57 eV uniformly, the relative separation of the excited electronic state
7230 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
Roszak et al. splitting of the I atom. Note that this peak is not>resolvablein the case of ClMMO since the C1 splitting is only 0.09 eV. The third peak is due to the removal of an electron from the C-X u-bonding orbital. This energy is roughly 4.0, 3.93, and 3.8 eV higher, respectively, than the energy required to form the ground state of the positive ion for HX+. We note that this interpretation is fully consistent with the observed spectra of oxetane-halogen derivatives. Acknowledgment. This research was supported, in part, by ONR under Contract N00014-80-C-0003. The calculations were carried out at the San Diego Supercomputer Center on the CRAY-YMP. We thank SDSC for an allocation of computer time.
References a d Notes (1) Kaufman, J. J.; Hariharan, P. C.; Roszak, S.;Ketgstra, P. B. In?. J. Quantum Chem., Quantum Chem. Symp. 1987,14, 37. (2)Kaufman, J. J.; Hariharan, P. C.; Keegstra, P. B. Int. J. Quantum Chem., Quantum Chem. Symp. 1987, 21,623. (3) Davidson, E. R.; Feller, D. Chem. Reo. 1986, 86, 681. (4)DeKoch, R. L.;Wong, W. S.;Fehlner, T. P. Inorg. Chem. 1983,21,
loniutlonPomntinl (ev)
Figure 4. He I spectrum for 3-bromomethyl-3-methyloxetane.
I 10.0
I
1
1
I
12.0
14.0
16 0
180
Ionization Potential (09
Figure 5. He I spectrum for 3-iodomethyl-3-methyloxetane.
and the spin-orbit splitting are computed with considerable accuracy. The 22:,2 excited state of HX’ is computed with greater accuracy compared to the 211 states as seen from Table XIII. The results of spin-orbit splittings and energy separations of the excited statej can be used to interpret the observed photoelectron spectra of halogenated oxetanes as follows. In the case of BrMMO (Figure 4) the fmt peak in the photoelectron spectrum corresponds to the removal of an electron from the predominantly nonbonding porbital on the oxygen atom. The second peaks 11, and IIb arc due to the spin-orbit splitting of the Br atom. In the case of IMMO (Figure 5) peaks I, and Ib are due to the spin-orbit
3203. (5) DeKoch, R. L.; Deshmukh, P.; Dutta, T. K.; Fehlner, T. P.; Housaxoft, C. E.; Hwang, J. L. S . Urganomerallics 1983,2, 1108. (6)Buenker, R. J.; Peyerimhoff, S.D. New Horizons of Quan?umChemistry; Lowdin, P.-O., Pullman, B., Eds.; Reidel: Dordrecht, 1983;p 183. (7)Buenker, R. J.; Peyerimhoff, S.D.; Butcher, W. Mol. Phys. 1983,35, 771. (8)(a) Buenker, R. J. Studies in Physical and Theoretical Chemistry; Carbo, R., Ed.; Elsevier Scientific: Amsterdam, 1982;Vol. 21, p 17. (b) Buenker. R. J. Proceedinm of Workshop on Ouantum Chemistrv and Molecular Physics; Wollongoig, ~ustralia,Feb. 160. (c) Buenker, R:J.; Philips, R. A. J . Mol. Struct. (THEOCHEM) 1985,123,291. (9)Buenker, R. J.; Peyerimhoff, S . D. Theor. Chim. Acta 1974,35, 33. (IO) (a) Buenker, R. J.; Peyerimhoff, S . D. Theor. Chin. Acta 1975.39, 217. (b) Knowlts, D. B.; Alvarez-Collado, J. R.; Hirsch, G.; Buenker, R. J. J . Chem. Phys. 1990, 92,585. ( I 1) Peyerimhoff, S.D.;Buenkcr, R. J. Chem. Phys. 1979.42, 167. (1 2) (a) Davidson, E. R. The World of Quantum Chemistry; Daudel, R., Pullman, B., Us.; Reidel: Dordrecht, 1979; p 17. (b) Langhoff, S. R.; Davidson, E. R. In?. J. Quantum Chem. 1974, 8, 61. (13)Paldus, J.; Wormer, P. E.; V i r , F.; van der Avoird, A. Proceedings of the 5th Seminar on Computational Methods in Quantum Chemistry; Groningen, The Netherlands, Sept 1983;p 13. (14) (a) Saunders, V. R.; Guest, M.F. Atmd 3 program System, Computational Science Group, SERC Dartsbury Laboratory. (b) Saunders, V. R.; van Hemert, M.;Hariharan, P. C. Atmol3 Program System adapted to the Cray XMP The Johns Hopkins University, 1985. ( 15) Buenker, R. J. MRD-CI F’rog” System, Lehrstuhl fur Theorctische Chemic, Gwmthochschule Wuppcrtal. (16)Schmidt, M.W.;Boatz, J. A,; Baldridge, K. K.; Koseki, S.;Gordon, M.S.;Elbert, S.T.; Lam, B. QCPE Bull. 1987,7. 115. (17)Pacios, L. F.; Christianecn, P. A. J . Chem. Phys. 1985,82, 2664. (18)Hurley, M.M.;Pacios, L. F.;Christianscn, P. A.; R w , R. B.; Ermler, W. C. J . Chem. Phys. 1986,84.6840. (19)LaJohn, L. A,; Christianecn, P. A.; Roes, R. B.; Atashroo, T.; Emler, W. C. J . Chem. Phys. 1987,87,2812. (20)van Duijneveldt, F. B. ZBM Tech. Res. Rep. 1971, RJ945. (21)Balasubramanian, K. J . Chem. Phys. 190,89,5731. (22)Balasubramanian, K. J . Chem. Phys. 1989,93,6585. (23)Huber, K. P.; Herzberg, G. Spectroscopic Constants of Diatomic Molecules; Van Nostrand: New York, 1979. (24)Huzinaga, S.J. Chem. Phys. 1%5,42,1923. Dunning, T. H., Jr. J . Chem. Phys. 1971,55,3958. (25)Gaussian Basis Sets for Molecular Calculations;Huzinaga, S., Ed.; Elsevier: Amsterdam. 1984. (26)Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry; Plenum: New York, 1977;Vol. 3, p 1. (27)Chapman, D. A.; Balasubramanian,K.; Lin, S . H.Phys. Reu. A 1988,
38,6098. (28) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand: New York, 1966;Vol. 3. (29)Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley-Interscience: London, 1970. (30) Brogli, F.;Heilbronner, E. Helu. Chim. Acta 1971,51, 1423.
