Absorption of SO2(g) by TDAE[O2SSO2](s) to Give TDAE[O2SS(O)2SO2](s): Related Reactions of [NR4]2[O2SSO2](s) (R = CH3, C2H5).

One mole equivalent of gaseous SO2 is absorbed by purple TDAE[O2SSO2](s), producing red, essentially spectroscopically pure TDAE[O2SS(O)2SO2](s); under prolonged evacuation, the product loses SO2(g), regenerating TDAE[O2SSO2](s). Similarly, [NR4]2[O2SS(O)2SO2](s) (R = Et, Me) can be prepared, albeit at lower purity, from the corresponding tetraalkylammonium dithionites (prepared by a modification of the known [NEt4]2[O2SSO2](s) preparation). While the [NEt4](+) salt is stable at rt; the [NMe4](+) salt has only limited stability at -78 °C. Vibrational spectra assignments for the anion in these salts were distinctly different from those for the anion in salts containing the long-known [O3SSSO3](2-) dianion, the most thermodynamically stable form of [S3O6](2-) (we prepared TDAE[O3SSSO3]·H2O(s) and obtained its structure by X-ray diffraction and vibrational analyses). The best fit between the calculated ((B3PW91/6-311+G(3df) and PBE0/6-311G(d)) and experimental vibrational spectra were obtained with the dianion having the [O2SS(O)2SO2](2-) structure. Vibrational analyses of the three [O2SS(O)2SO2](2-) salts prepared in this work showed that the corresponding [O3SSO2](2-) salts were present as a ubiquitous decomposition product. The formation of these new [O2SS(O)2SO2](2-) dianion salts was predicted to be favorable for [NMe4](+) and larger cations using a combination of theoretical calculations (B3PW91/6-311+G(3df)) and volume based thermodynamics (VBT). Similar methods accounted for the greater stabilities of the TDAE(2+) and [NEt4](+) salts of [O2SS(O)2SO2](2-) compared to [NMe4]2[O2SS(O)2SO2](s) toward irreversible decomposition to the corresponding [O3SSO2](2-) salts. These salts represent the first known examples of a new class of poly(sulfur dioxide) dianion, [SO2]n(2-) in which n > 2.

Synthesis of [N(CH3)4]2O3SOSO2(s) and [N(CH3)4]2[(O2SO)2SO2]·SO2(s) containing (SO4)(SO2)x(2-) x = 1, 2, members of a new class of sulfur oxydianions.

One mole equivalent of SO2 reversibly reacts with [N(CH3)4]2SO4(s) to give [N(CH3)4]2S2O6(s) (1) containing the [O3SOSO2](2-), shown by Raman and IR to be an isomer of the [O3SSO3](2-) dianion. The experimental and calculated (B3PW91/6-311+G(3df)) vibrational spectra are in excellent agreement, and the IR spectrum is similar to that of the isoelectronic O3ClOClO2. Crystals of [N(CH3)4]2(O2SO)2SO2·SO2 (2) were isolated from solutions of [N(CH3)4]2SO4 in liquid SO2. The X-ray structure showed that 2 contained the [(O2SO)2SO2](2-) dianion. The characterized N(CH3)4(+) salts 1 and 2 are the first two members of the (SO4)(SO2)x(2-) class of sulfur oxydianions analogous to the well-known small cation salts of the SO4(SO3)x(2-) polysulfates.

Synthesis of (TDAE)(O2SSO2)(s) and discovery of (TDAE)(O2SSSSO2)(s) containing the first polythionite, [O2SSSSO2]2-.

Gaseous SO2 reacts with tetrakis(dimethylamino)ethylene (TDAE) in acetonitrile in a 2:1 stoichiometric ratio to give analytically pure insoluble purple (TDAE)(O2SSO2) (1) in about 80% yield. Crystals of (TDAE)(O2SSSSO2) (2) were obtained from orange solution over the purple solid. The Raman spectrum of [TDAE](2+) was established using (TDAE)(A) salts [A = 2Br(-), 2Br(-)·2H2O (X-ray), 2[Br3](-) (X-ray)]. Vibrational spectroscopy showed that [O2SSO2](2-) in 1 has C2h geometry. The X-ray structure of 2 showed that it contained [O2SSSSO2](2-), the first example of a new class of sulfur oxyanions, the polythionites. The geometry of [O2SSSSO2](2-) consists of S2 with an S-S bond length of 2.003(1) Å connected to two terminal SO2 moieties by much longer S-S bonds of 2.337(1) Å. Calculations (B3PW91/6-311+G(3df)) show that the structural units in [O2SSSSO2](2-) are joined by the interaction of electrons in two mutually perpendicular π* SOMOs of the triplet-state diradical S2 with unpaired electrons in the π*-antibonding orbitals of the two terminal [SO2](•-) and polarized to delocalize the negative charge equally onto the three fragments. Thermodynamic estimates show 2 to be stable with respect to loss of sulfur and formation of 1, in contrast to [O2SSSSO2](2-) salts of small cations that are unstable toward the related dissociation. Reaction of TDAE with an excess of liquid SO2 led to (TDAE)(O3SOSO3)·SO2 (preliminary X-ray, Raman), (TDAE)(O3SSSSO3)·2SO2 (preliminary X-ray, Raman), and (TDAE)(O3SSO2) (Raman).

Pseudo Jahn-Teller coupling in trioxides XO3((0,1,-1)) with 22 and 23 valence electrons.

D3h and C2v geometries and energies, vertical excitation energies, as well as minimal energy paths as function of the O(1)(z)-X-O(2) angle α were obtained for XO3((0,1,-1)) (X = B, Al, Ga; C, Si, Ge; N, P, As; S, Se) molecules and ions with 22 and 23 valence electrons (VE), using density functional theory (DFT), coupled cluster with single and double substitutions with noniterative triple excitations (CCSD(T)), equation of motion (EOM)-CCSD, time-dependent DFT, and multi-reference configuration interaction methods. It is shown that pseudo Jahn-Teller (PJT) coupling increases as the central atom X becomes heavier, due to decreases in excitation energies. As is well known for CO3, the excited (1)E' states of the 22 VE systems SiO3, GeO3; NO3(+), PO3(+), AsO3(+); BO3(-), AlO3(-), GaO3(-) have strong vibronic coupling with the (1)A1' ground state via the e' vibrational modes, leading to a C2v minimum around α = 145°. For first and second row X atoms, there is an additional D3h minimum (α = 120°). Interacting excited states have minima around 135°. In the 23 VE systems CO3(-), SiO3(-); NO3, PO3; SO3(+), coupling of the excited (2)E' with the (2)A2' ground state via the e' mode does not generate a C2v state. Minima of interacting excited states are close to 120°. However, due to very strong PJT coupling, a double-well potential is predicted for GeO3(-), AsO3, and SeO3(+), with a saddle point at D3h symmetry. Interaction of the b2 highest occupied molecular orbital with the b2 lowest unoccupied molecular orbital, both oxygen lone pair molecular orbitals, is seen as the reason for the C2v stabilization of 22 VE molecules.

Structures and vibrational spectra of SO(n)(p-) sulfur oxides, MSO(n)(-) anions, and MSO(n), M2SO(n) salts in the gas phase (n = 1-3; p = 0-2; M = Li, Na, K). A density functional theory study.

This theoretical study focuses on geometries, vibrational spectra, charge distributions, electron affinities, and reaction energies for SO(n)(p-) anions and alkali salts MSO(n)(-), M(1,2)SO(n) in the gas phase (n = 1-3; p = 0-2; M = Li-K). Most of our data for compounds with the S oxidation states 0, 2, and 4 are new in the literature. The bulk of the results are obtained at the B3PW91 level, with CCSD(T)=FC calculations carried out for relative energy calibrations; the 6-311+G(3df) basis set is used throughout. The formation of contact ion pairs is prevalent; they are of type: (i) M(+)(SO(n)(-)) for the π-radicals MSO, MSO(2), MSO(3) of doublet multiplicity; (ii) (M(+))(2)(SO(n)(2-)) for M(2)SO, M(2)SO(2), M(2)SO(3) in their singlet ground states; and (iii) M(ns)(SO(n)(-)) for the radicals MSO(-), MSO(2)(-), MSO(3)(-) in their triplet states. When isolated in matrices, M(2)SO and M(2)SO(2) will facilitate the spectroscopic study of the little known SO(2-) and SO(2)(2-) ions. Divalent M(2)SO(n) salts, due to their large dipole moments, should be highly soluble in polar solvents, first dissociating into MSO(n)(-) + M(+) products. For MSO(3), bidentate coordination OS(O(2)M) is preferred over tridentate S(O(3)M) binding. We confirm that all MSO(2) molecules are planar, at variance with an ESR study assigning to NaSO(2) a nonplanar structure. This study partially support the assignment of an experimental frequency at 918.2 cm(-1) (932 cm(-1), calculated) to the antisymmetric ν(a)(SO) mode of the elusive sulfoxilate ion, SO(2)(2-). A definitive identification, however, would require to record the vibrational spectrum below 800 cm(-1) (apparently not done in the original work) because the missing symmetric ν(s)(SO) mode is here found to lie around 760 cm(-1), exhibiting high intensity in both IR and Raman spectra.

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies.

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.

Preparation and characterization of (CNSSS)2(A)2 (A = AsF6(-), SbF6(-), Sb2F11(-)) containing the O2-like 5,5'-bis(1,2,3,4-trithiazolium) dication: the second example of a simple nonsterically hindered main-group diradical that retains its paramagnetism in the solid state.

The reaction of NC-CN with a 1:1 mixture of S(4)(MF(6))(2) and S(8)(MF(6))(2) (M = As, Sb) (stoichiometrically equivalent to four "S(3)MF(6)" units) results in the quantitative formation of S(3)NCCNS(3)(MF(6))(2) [7(MF(6))(2)], which is the thermodynamic sink in this reaction. The Sb(2)F(11)(-) salt 7(Sb(2)F(11))(2) is prepared by the addition of an excess of SbF(5) to 7(AsF(6))(2). Crystal structure determinations for all three salts show that 7(2+) can be viewed as two R-CNS(3)(+) radical cations joined together by a C-C single bond. The two rings are coplanar and in a trans orientation due to electrostatic N(delta-)...S(delta+) interactions. The classically bonded alternative (quinoidal structure), in which the octet rule is obeyed, is not observed and is much higher in energy based on calculated estimates and a simple comparison of pi bond energies. Calculated molecular orbitals (MOs) support this, showing that the MO corresponding to the quinoidal structure lies higher in energy than the nearly degenerate singly occupied MOs of 7(2+). The vibrational spectra of 7(2+) in all salts were assigned based on a normal-coordinate analysis and theoretical vibrational frequencies calculated at the PBE0/6-31G* level. In the solid state, 7(2+) is a planar disjoint diradical with essentially degenerate open-shell singlet and triplet states. The disjoint nature of the diradical cation 7(2+) is established by magnetic susceptibility studies of the Sb(2)F(11)(-) salt doped in an isomorphous diamagnetic host material (CNSNS)(2)(Sb(2)F(11))(2) [10(Sb(2)F(11))(2)]. Intramolecular spin coupling is extremely weak corresponding to a singlet-triplet gap (DeltaE(ST) = 2J) of <+/-2 cm(-1). CASPT2[12,12]/6-311G* calculations support a triplet ground state with a small singlet-triplet gap. The single-crystal electron paramagnetic resonance (EPR) of 7(Sb(2)F(11))(2) doped in 10(Sb(2)F(11))(2) is in agreement with the triplet state arising from the weak coupling between the unpaired electrons residing in p(pi) orbitals in each of the rings. Variable-temperature susceptibility data for bulk samples of 7(A)(2) (A = SbF(6)(-), AsF(6)(-), Sb(2)F(11)(-)) are analyzed by employing both 1D chain and 2D sheet magnetic models. These studies reveal significant intermolecular exchange approximating that of a 1D chain for the SbF(6)(-) salt with |J| = 32 cm(-1). The exchange coupling is on the same order of magnitude as that for the AsF(6)(-) salt, although in this case it is likely that there are complex exchange pathways where no particular one is dominant. Intermolecular exchange in the Sb(2)F(11)(-) salt is an order of magnitude weaker. In solution, the EPR spectrum of 7(2+) shows a broad triplet resonance as well as a sharp resonance that is tentatively attributed to a rotomer of the 7(2+)/anion pair, which is likely the origin of the green species given on dissolution of the red 7(2+) salts in SO(2)/AsF(3)/MF(5). We account for the many similarities between O(2) and 7(2+), which are the only simple nonsterically hindered nonmetal diradicals to retain their paramagnetism in the solid state. 7(2+) is also the first isolable, essentially sulfur-based diradical as evidenced by calculated spin densities.

Excited states of dibromine monoxide (Br2O): MRCI, coupled cluster, and density functional studies.

Vertical excitation energies up to about 9 eV, and related oscillator strengths, were calculated by multireference configuration interaction (MRCI) methods for singlet and triplet states of BrOBr in C(2v) symmetry, including the low-lying s- and p-Rydberg states. Observed maxima in the visible/UV spectra were identified as excitations to 1(3)B(1) (665 nm, 1.86 eV), 1(1)B(1) (520 nm, 2.38 eV), 1(1)B(2) (355 nm, 3.49 eV), 2(1)A(1) (314 nm, 3.94 eV), and 3(1)B(2) (approximately 200 nm, approximately 6.20 eV). The calculated vertical excitation energies lie within 0.1 eV of the observed values. Many more singlet and weaker triplet excitations are predicted. Although most excited states have small oscillator strengths, that of 3(1)B(2) is very large. Vertical excitation energies were also calculated at the 1(1)A' ground state geometry of the BrBrO isomer. Using DFT/B3LYP and CCSD(T) (CC) methods with the 6-311+G(3df) basis set, geometries were optimized for about 12 excited singlet and triplet states of BrOBr in C(2v) symmetry. Frequency analysis showed that many states, including 1(1)B(1), 1(1)B(2), 1(3)B(1), and 1(3)B(2), are not stable. C(s) structures corresponding to 1(1)B(1), 1(3)B(1), and 1(3)B(2) were optimized. In addition, geometry optimizations were performed for the lowest singlet and triplet A' and A'' states of BrBrO. This isomer lies 0.61 (CC) to 0.66 eV (MRCI) above BrOBr. Comparison was made with the lowest excited states of Cl(2)O and F(2)O.

The electronic spectrum and photodissociation of dinitrogen tetroxide (N(2)O(4)): Multireference configuration interaction studies.

Multireference configuration interaction (MRCI) calculations were performed for vertical excitation energies and potential curves of N(2)O(4) in D(2h) symmetry using the TZVPP basis set with diffuse functions on the nitrogens. The strong absorption of N(2)O(4) around 185 nm is assigned to the transition from the ground state to 1 (1)B(1u) (σ(O)→σ(∗) (N-N)) rather than 1 (1)B(2u) (π(O)→π(∗) (NO(2) ),n→σ(∗) (N-N)), as previously assumed. (N(2)O(4) is placed in the yz-plane, with N-N along z.) Transition to 1 (1)B(1u) is calculated to have an oscillator strength f=0.71 and is z-polarized, in agreement with the experimental observations. Another state, 2 (1)B(2u), lies close by, however, at a much lower f-value. The weak absorption around 340 nm is assigned to 1 (1)B(3u). Excitation to 1 (1)B(2u) is calculated at 227 nm. There is no clear assignment of a state for the observed shoulder around 260 nm. TD-DFT (time-dependent density functional theory) vertical excitation energies are close to MRCI values. MRCI singlet and triplet potential curves for the dissociation N(2)O(4)→2NO(2), combined with a table of NO(2) states correlating with those of N(2)O(4), indicate possible products of photodissociation at various wavelengths. The extensive literature on the photodissociation of N(2)O(4) is reviewed. DFT geometry optimizations have been performed on low-lying singlet and triplet states.

Axial asymmetry of the charge- and spin-density distributions in Pi states. Molecular quadrupole moments and hyperfine coupling constants of CH, NH, OH, CF, LiO, NO, and FO.

The axial asymmetry of the charge- and spin-density distributions in Pi states is studied via second-rank traceless tensors P(ii) (ii = xx, yy, zz), namely, quadrupole moments (Theta(ii)), electric field gradients (q(ii)), and magnetic dipolar (T(ii)) hyperfine coupling constants (hfcc's). In linear molecules, it holds that P(xx) does not = P(yy) does not = P(zz) for Pi, but P(xx) = P(yy) does not = P(zz) for Sigma, Delta, Phi,..., states. Thus, traceless P(ii) in Pi states have two independent parameters, P(parallel) = P(zz) is proportional to [r(m)(3 cos2 theta - 1] and deltaP(perpendicular) = |P(xx) - P(yy)| is proportional to [r(m) sin2 theta], with m = 2(Theta(ii)) or -3(q(ii), T(ii)). All linear states have P(parallel) does not = 0, but only Pi states exhibit deltaP(perpendicular) does not = 0, as shown by hfcc's like c = (3/2)T(zz), and d = |T(xx) - T(yy)|, as well as q0 = (-q(zz)) and |q(2)/2| = |q(xx) - q(yy)|. Little is known about Theta(zz) and deltaTheta(perpendicular) = |Theta(xx) - Theta(yy)| in Pi states since most experimental values (gas-phase) are rotational averages, and several theoretical studies have reported Theta(zz) but assumed deltaTheta(perpendicular) = 0. The diatomics studied here have X2Pi(1/2)(pi1) ground states, like CH and NO, or are of type X2Pi(3/2)(pi3), like OH, CF, LiO, and FO. The A3Pi(sigma pi3) state of NH is also included. Our P(parallel) and deltaP(perpendicular) values--calculated at the experimental R(e)'s with the B3LYP/aug-cc-pVQZ method--reproduce well the available literature data. The properties of the CF and FO radicals are not well-known so that our {c, d} and {q0, q2} values should help future experimental studies of their hyperfine spectra. Excluding OH, the complete quadrupole sets {Theta(zz), deltaTheta(perpendicular)} are new for all species discussed here. For comparison purposes, Theta(zz) of a low-lying Sigma state is also calculated for each X2Pi radical.

Coupled cluster and density functional studies on geometries and energies of excited C(2v) states of ozone.

The performance of single-determinant methods for finding geometries and energies of excited states is tested on the ozone molecule. Geometries for low-lying singlet and triplet states of ozone were optimized by CCSD(T) and density functional theory (DFT) (with BPW91 functional) methods. DFT geometries were found to lie close to CCSD(T) values. Most CCSD(T) and DFT geometries and energies are in good agreement with available experimental and recent high-level theoretical values, with deviations lying within 0.02 A, 2 degrees, and 0.3 eV. An exception is the 1 (1)B(2) state, having a larger deviation of bond distance and energy. A multiconfigurational treatment is required for this state. DFT geometry optimizations and calculations of vibrational frequencies were extended to higher states, covering over 30 excited states of ozone, with adiabatic excitation energies up to about 6 eV. Calculated harmonic frequencies showed several states, including 1 (1)B(2), to be saddle points. Multireference configuration interaction (MRCI) bending potentials for first and second singlet and triplet states were used in verifying the CCSD(T) and DFT geometries and for locating additional minima. For first states, DFT bending potentials are compared with MRCI potentials. As a criterion for the quality of single-determinant geometries and energies of excited states, comparison of their vertical excitation energies with MRCI or time-dependent DFT values is recommended.

N-heterocyclic carbene complexes of Rh: reaction with dioxygen without oxidation.

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d8 metal complex, rather than the more common Rh(III) d6 peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 A range.

NC-(CF2)4-CNSSN radical containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting-solidification: structural, spectroscopic, and magnetic characterization.

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC-(CF2)4-CNSSN radical by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric Sdelta+...N(delta-) interactions of ca. 3.2 angstroms, and the dimeric units are linked by electrostatic -C triple bond N(delta-)...Sdelta+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 microB per dimer (1.3 microB per monomer) at 300 K and a good fit to the Bleaney-Bowers model in the temperature range 2-300 K with 2J = -1500 +/- 50 cm(-1), g = 2.02(5), rho = 0.90(3)%, and TIP = 1.25(4) x 10(-3) emu mol(-1). The [NC-(CF2)4-CNSSN radical]2 dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = -1730 +/- 100 cm(-1), |D| = 0.0278(5) cm(-1), |E| = 0.0047(5) cm(-1)]. The value of the singlet-triplet gap has enabled us to estimate the "in situ" dimerization energy of the radical dimer as ca. -10 kJ mol(-1). The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305-335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown (paramagnetic liquid).

Density functional theory and multireference configuration interaction studies on low-lying excited states of TiO2.

Using density functional theory at the BPW916-311+G(3df) level, optimized geometries and energies of the lowest singlet, triplet, and quintet A(1), A(2), B(1), B(2)(C(2v)) states of the TiO(2) molecule were obtained. TiO(2) has a (1)A(1) ground state in C(2v) symmetry. Adiabatic excitation energies of the low-lying singlet and triplet states range from 2.1 to 3.0 eV. The (1,3)A(2) states optimize at bond angles of about 140 degrees , lying only 0.06 eV below linear (1,3)Delta(u), whereas (1,3)B(1) and (1,3)B(2), with bond angles of 120 degrees and 96 degrees , respectively, lie 0.3-0.4 eV below the respective (1,3)Pi(u) or (1,3)Delta(u) states. Minima with short O-O distances of approximately 1.46 A, at energies of 4.2 and 4.7 eV, were found for (1)A(1) and (3)A(1). The C(2v) minima of the lowest (1)B(1) and (3)B(1) states are saddle points, suggesting lower-energy structures in C(s) symmetry. The C(2v) quintet states start at energies of 5.7 eV. Multireference configuration interaction (MRCI) methods, employing a polarized valence triple-zeta basis set, lead to similar geometries and energies. MRCI vertical excitation energies up to 4.6 eV and oscillator strengths are given. The calculated excitation energy of 2.2 eV for (1)B(2) agrees well with 2.3 eV from a fluorescence spectrum. The vertical electron detachment energy of TiO(2) (-) is 1.5 eV, in good agreement with 1.6 eV from anion photoelectron spectroscopy. An observed second photoelectron band corresponds to (1)B(2) and/or (3)B(2), but the assignment of a third band could not be verified. Vibrational frequencies, ionization energies, electron affinities, and dissociation energies are given.

Quadrupole, octopole, and hexadecapole electric moments of Sigma, Pi, Delta, and Phi electronic states: cylindrically asymmetric charge density distributions in linear molecules with nonzero electronic angular momentum.

The number of independent components, n, of traceless electric 2(l)-multipole moments is determined for C(infinity v) molecules in Sigma(+/-), Pi, Delta, and Phi electronic states (Lambda=0,1,2,3). Each 2(l) pole is defined by a rank-l irreducible tensor with (2l+1) components P(m)((l)) proportional to the solid spherical harmonic r(l)Y(m)(l)(theta,phi). Here we focus our attention on 2(l) poles with l=2,3,4 (quadrupole Theta, octopole Omega, and hexadecapole Phi). An important conclusion of this study is that n can be 1 or 2 depending on both the multipole rank l and state quantum number Lambda. For Sigma(+/-)(Lambda=0) states, all 2(l) poles have one independent parameter (n=1). For spatially degenerate states--Pi, Delta, and Phi (Lambda=1,2,3)--the general rule reads n=1 for l<2/Lambda/ (when the 2(l)-pole rank lies below 2/Lambda/ but n=2 for higher 2(l) poles with l>or=2/Lambda/. The second nonzero term is the off-diagonal matrix element [formula: see text]. Thus, a Pi(Lambda=1) state has one dipole (mu(z)) but two independent 2(l) poles for l>or=2--starting with the quadrupole [Theta(zz),(Theta(xx)-Theta(yy))]. A Delta(Lambda=2) state has n=1 for 2((1,2,3)) poles (mu(z),Theta(zz),Omega(zzz)) but n=2 for higher 2((l>or=4)) poles--from the hexadecapole Phi up. For Phi(Lambda=3) states, it holds that n=1 for 2(1) to 2(5) poles but n=2 for all 2((l>or=6)) poles. In short, what is usually stated in the literature--that n=1 for all possible 2(l) poles of linear molecules--only applies to Sigma(+/-) states. For degenerate states with n=2, all Cartesian 2(l)-pole components (l>or=2/Lambda/) can be expressed as linear combinations of two irreducible multipoles, P(m=0)((l)) and P/m/=2 Lambda)((l)) [parallel (z axis) and anisotropy (xy plane)]. Our predictions are exemplified by the Theta, Omega, and Phi moments calculated for Lambda=0-3 states of selected diatomics (in parentheses): X (2)Sigma(+)(CN), X (2)Pi(NO), a (3)Pi(u)(C(2)), X (2)Delta(NiH), X (3)Delta(TiO), X (3)Phi(CoF), and X (4)Phi(TiF). States of Pi symmetry are most affected by the deviation from axial symmetry.

Theoretical studies on dications and trications of FH, ClH, and BrH. Properties of the bound 1(5)sigma- states. Electron-spin g-factors and fine/hyperfine constants of the metastable X3Sigma- states of ClH2+ and BrH2+.

This theoretical study reports calculations on the fine and hyperfine structure parameters of the metastable X(3)Sigma(-)(sigma(2)pi(2)) state of ClH(2+) and BrH(2+). Data on the repulsive FH(2+) system are also included for comparison purposes. The hyperfine structure (hfs) coupling constants for magnetic (A(iso), A(dip)) and quadrupole (eQq) interactions are evaluated using B3LYP, MP4SDQ, CCSD, and QCISD methods and several basis sets. The fine structure (fs) constants (zero-field splitting lambda and spin-rotation coupling gamma) and electron-spin magnetic moments (g-factor) are evaluated in 2nd-order perturbation theory using multireference CI (MRCI) wave functions. Our calculations find for (35)Cl of ClH(2+) A(iso)/A(dip) = 110/-86 MHz; eQq(0) = -59 MHz; 2lambda = 20.4 cm(-1); g( perpendicular)(v = 0) = 2.02217; and gamma = -0.31 cm(-1) (to be compared with the available experimental A(iso)/A(dip)= 162/-30 MHz). For (79)BrH(2+), the corresponding values are 300/-400 MHz; 368 MHz; 362.6 cm(-1); 2.07302; and -0.98 cm(-1) (experimental 2lambda = 445(+/-80) cm(-1)). We find g( perpendicular)(ClH(2+)) to increase by about 0.0054 between v = 0 and 2, whereas the experimental effective g( perpendicular) changes drastically with vibrational excitation. Nuclear quadrupole coupling constants for halogen atoms X are found to be as large as corresponding A(dip)(X)'s, indicating that both terms may have to be included in the Hamiltonian used to interpret XH(2+) hyperfine spectra. A novel finding relates to the bound character of the 1(5)Sigma(-)(sigmapi(2)sigma) state in FH(2+), as already known for ClH(2+) and BrH(2+), but having a deeper potential well D(e) approximately 4,000 cm(-1) (versus 1,000 cm(-1) in the heavier radicals). Vertical ionization potentials for formation of XH(3+) trications are also discussed.

Theoretical and experimental studies on the Baeyer-Villiger oxidation of ketones and the effect of alpha-halo substituents.

The Baeyer-Villiger reactions of acetone and 3-pentanone, including their fluorinated and chlorinated derivatives, with performic acid have been studied by ab initio and DFT calculations. Results are compared with experimental findings for the Baeyer-Villiger oxidation of aliphatic fluoro and chloroketones. According to theoretical results, the first transition state is rate-determining for all substrates even in the presence of acid catalyst. Although the introduction of acid into the reaction pathway leads to a dramatic decrease in the activation energy for the first transition state (TS), once entropy is included in the calculations, the enthalpic gain is lost. Of all substrates examined, pentanone reacts with performic acid via the lowest energy transition state. The second transition state is also lowest for pentanone, illustrating the accelerating effect of the additional alkyl group. Interestingly, there is only a small energetic difference in the transition states leading to migration of the fluorinated substituent versus the alkyl substituent in fluoropentanone and fluoroacetone. These differences match remarkably well with the experimentally obtained ratios of oxidation at the fluorinated and nonfluorinated carbons in a series of aliphatic ketones (calculated, 0.3 kcal/mol, observed, 0.5 kcal/mol), which are reported herein. The migration of the chlorinated substituent is significantly more difficult than that of the alkyl, with a difference in the second transition state of approximately 2.6 kcal/mol.

Rare gas effects on hyperfine coupling constants of BO, AlO, and GaO.

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.

Electron spin resonance g tensors for complexes of Ne and Ar with AlO: theoretical studies related to the large matrix effect observed for AlO.

For Ne(n)-AlO (n=2, 4, 6, 8, 10) and Ar(n)-AlO clusters (n=2, 4, 6, 8), the perpendicular (relative to AlO) component of the g tensor was calculated by second-order perturbation theory, using multireference configuration-interaction wave functions. The rare-gas (Rg) atoms were placed axially and/or off axially (one or two rings of four Rg atoms each), and the distance of the Rg atoms from the Al and O atoms, or from the AlO axis, was varied from 4 to 12 bohrs. Rg atoms placed axially mostly increase g(perpendicular), whereas off-axially placed ones lower it below the gas-phase value of AlO. The largest deviations from g(perpendicular) of isolated AlO occur at Ne-Al,O distances of 5-6 bohrs, and Ar-Al,O distances of 6-9 bohrs, with maximal lowerings of about 1600 ppm for Ne and about 2200 ppm (estimated) for Ar in the case of two axial and eight off-axial Rg atoms. Electron spin resonance studies by Knight and Weltner found large matrix effects for AlO, with downshifts of g(perpendicular) observed to be about 450 and 1150 ppm in Ne and Ar matrices, respectively.