A multireference coupled-cluster potential energy surface of diazomethane, CH(2)N(2)

The intrinsically multireference dissociation of the C-N bond in ground-state diazomethane (CH(2)N(2)) at different angles has been studied with the multireference Brillouin-Wigner coupled-cluster singles and doubles (MRBWCCSD) method. The morphology of the calculated potential energy surface (PES) in C(s)() symmetry is similar to a multireference perturbational (CASPT3) PES. The MRBWCCSD/cc-pVTZ H(2)C-N(2) dissociation energy with respect to the asymptotic CH(2)(?(1)A(1)) + N(2)(X(1)Sigma(g)(+)) products is D(e) = 35.9 kcal/mol, or a zero-point corrected D(0) = 21.4 kcal/mol with respect to the ground-state CH(2)(X(3)B(1)) + N(2)(X(1)Sigma(g)(+)) fragments.     

Infrared laser spectroscopy of CH3...HF in helium nanodroplets: The exit-channel complex of the F + CH4 reaction

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.     

Mode-specific tunneling dynamics in the ground electronic state of tropolone

The mode specificity of proton-transfer dynamics in the ground electronic state (X (1)A(1)) of tropolone has been explored at near-rotational resolution by implementing a fully coherent variant of stimulated emission pumping within the framework of two-color resonant four-wave mixing spectroscopy. Three low-lying (E(vib) approximately 550-750 cm(-1)) vibrational features, assigned to nu(30)(a(1)), nu(32)(b(2)), and nu(31)nu(38)(a(1)), have been interrogated under ambient, bulk-gas conditions, with term energies determined for the symmetric and antisymmetric (tunneling) components of each enabling the attendant tunneling-induced bifurcations of 1.070(9), 0.61(3), and 0.07(2) cm(-1) to be extracted. The dependence of tunneling rate (or hydron migration efficiency) on vibrational motion is discussed in terms of corresponding atomic displacements and permutation-inversion symmetries for the tropolone skeleton.     

Thermochemistry of key soot formation intermediates: C3H3 isomers

Accurate standard enthalpies of formation for allene, propyne, and four C3H3 isomers involved in soot formation mechanisms have been determined through systematic focal point extrapolations of ab initio energies. Auxiliary corrections have been applied for anharmonic zero-point vibrational energy, core electron correlation, the diagonal Born-Oppenheimer correction (DBOC), and scalar relativistic effects. Electron correlation has been accounted for via second-order Z-averaged perturbation theory (ZAPT2) and primarily through coupled-cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [ROCCSD, ROCCSD(T), ROCCSDT, and UCCSDT(Q)]. The correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, 6), was employed. The CCSDT(Q) corrections do not exceed 0.12 kcal mol(-)1 for the relative energies of the systems considered here, indicating a high degree of electron correlation convergence in the present results. Our recommended values for the enthalpies of formation are as follows: Delta(f)H(o)(0)(propargyl) = 84.76, Delta(f)H(o)(0) (1-propynyl) = 126.60, Delta(f)H(o)(0) (cycloprop-1-enyl) = 126.28, Delta(f)H(o)(0)(cycloprop-2-enyl) = 117.36, Delta(f)H(o)(0)(allene) = 47.41, and Delta(f)H(o)(0)(propyne) = 46.33 kcal mol(-1), with estimated errors no larger than 0.3 kcal mol(-1). The corresponding C3H3 isomerization energies are about 1 kcal mol(-1) larger than previous coupled-cluster results and several kcal mol(-1) below those previously obtained using density functional theory.     

Ab initio study of the influence of trimer formation on one- and two-bond spin-spin coupling constants across an X-H-Y hydrogen bond: AH:XH:YH3 complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.     

The ab initio limit quartic force field of BH3

The complete quartic force field of BH(3) has been converged to the ab initio limit by extrapolation of core-valence correlation-consistent basis set series (cc-pCVXZ, X = T, Q, 5) of all-electron CCSD(T) (coupled-cluster singles and doubles with perturbative triples) energy points. Additional computations including full coupled-cluster treatments through quadruple excitations (CCSDTQ), scalar relativistic effects, and diagonal Born-Oppenheimer corrections (DBOC) were concurrently executed. Within second-order vibrational perturbation theory (VPT2) our quartic force field yields the fundamental frequencies nu(1) = 2502.3 cm(-1), nu(2) = 1147.2 cm(-1), nu(3) = 2602.1 cm(-1), and nu(4) = 1196.5 cm(-1), in excellent agreement with observed gas-phase fundamentals, displaying a mean absolute error of only 0.3 cm(-1). Our converged prediction for the equilibrium bond length of BH(3) is r(e) = 1.1867 A.     

The Beryllium tetramer: profiling an elusive molecule

The structure and energetics of Be(4) are investigated using state-of-the-art coupled-cluster methods. We compute the optimized bond length, dissociation energy, and anharmonic vibrational frequencies. A composite approach is employed, starting from coupled-cluster theory with single, double, and perturbative triple excitations extrapolated to the complete basis set (CBS) limit using Dunning's correlation consistent cc-pCVQZ and cc-pCV5Z basis sets. A correction for full triple and connected quadruple excitations in the smaller cc-pCVDZ basis set is then added, yielding an approximation to CCSDT(Q)/CBS denoted c～CCSDT(Q). Corrections are included for relativistic and non-Born-Oppenheimer effects. We obtain D(e) = 89.7 kcal mol(-1), D(0) = 84.9 kcal mol(-1), and r(e) = 2.043 A?. Second-order vibrational perturbation theory (VPT2) is applied to a full quartic force field computed at the c～CCSDT(Q) level of theory, yielding B(e) = 0.448 cm(-1) and fundamental frequencies of 666 (a(1)), 468 (e), and 571 (t(2)) cm(-1). Computations on the spectroscopically characterized Be(2) molecule are reported for the purpose of benchmarking our methods. Perturbative estimates of the effect of quadruple excitations are found to be essential to computing accurate parameters for Be(2); however, they seem to exert a much smaller influence on the structure and energetics of Be(4). Our extensive characterization of the Be(4) bonding potential energy surface should aid in the experimental identification of this thermodynamically viable but elusive molecule.     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

The highly anharmonic BH5 potential energy surface characterized in the ab initio limit

The strong sensitivity to level of theory of the salient features of the ground state potential energy surface of BH(5) has been overcome by rigorously converged ab initio computations employing correlation-consistent basis sets cc-p(C)VXZ (X=2-6), explicitly correlated R12 corrections, and coupled-cluster theory complete through quadruple excitations (CCSDTQ). Extrapolations via our focal point method yield a C(s)-symmetry global minimum of BH(3)-H(2) type featuring interfragment B-H distances of (1.401, 1.414) A, an H(2) bond length elongated to 0.803 A, and a BH(3)+H(2) dissociation energy D(e)(D(0))=6.6 (1.2) kcal mol(-1). The classical barriers for H(2) internal rotation and hydrogen scrambling are 0.07 and 5.81 kcal mol(-1), respectively, above the BH(5) minimum. Our thermochemical computations yield Delta(f)H(0) ( degrees )[BH(5)(g)]=-111.3+/-0.2 kcal mol(-1)+Delta(f)H(0) ( degrees )[B(g)], which is limited in accuracy only by persistent uncertainties in the enthalpy of formation of gaseous boron. As a first step in investigating the extremely anharmonic 12-dimensional vibrational dynamics of BH(5), a complete quartic force field has been computed at the all-electron cc-pCVQZ CCSD(T) level of theory. Previous matrix isolation infrared assignments of the nu(2) and nu(9) stretching modes of BH(5) compare favorably with our computed vibrational fundamentals, but the experimental assignment of the nu(6) bending mode of the BH(3) moiety is not supported by computed isotopic shifts.     

Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

The singlet ground ((approximate)X(1)Sigma1+) and excited (1Sigma-,1Delta) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum ((approximate)X(1)Sigma+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of omega1=3344 cm(-1), omega2=689 cm(-1), and omega3=1298 cm(-1). Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol(-1) above the global minimum HCP. The dissociation energy D0[HCP((approximate)X(1)Sigma+)-->H(2S)+CP(X2Sigma+)] of HCP is predicted to be 119.0 kcal mol(-1), which is very close to the experimental lower limit of 119.1 kcal mol(-1). Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Sigma- -->1A", with excitation energies Te of 101.4 kcal mol(-1) (1A"HCP), 104.6 kcal mol(-1)(1Sigma-HCP), 122.3 kcal mol(-1)(1A" HPC), and 171.6 kcal mol(-1)(1Sigma-HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A" state with a predicted bond angle of 129.5 degrees compare well with the experimentally reported first singlet state ((approximate)A1A"). The excitation energy predicted for this excitation is T0=99.4 kcal mol(-1) (34 800 cm(-1),4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol(-1)(34 746 cm(-1),4.308 eV). For the second lowest-lying excited singlet surface, 1Delta-->1A', four stationary points were found with Te values of 111.2 kcal mol(-1) (2(1)A' HCP), 112.4 kcal mol(-1) (1Delta HPC), 125.6 kcal mol(-1)(2(1)A' HCP), and 177.8 kcal mol(-1)(1Delta HPC). The predicted CP bond length and frequencies of the 2(1)A' state with a bond angle of 89.8 degrees (1.707 A, 666 and 979 cm(-1)) compare reasonably well with those for the experimentally reported (approximate)C(1)A' state (1.69 A, 615 and 969 cm(-1)). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol(-1) and 89.8 degrees versus experimental values of 115.1 kcal mol(-1) and 113 degrees. of 115.1 kcal mol(-1) and 113 degrees.     

Inversion vibration of PH3+(X2A2") studied by zero kinetic energy photoelectron spectroscopy

We report the first rotationally resolved spectroscopic studies on PH3+(X2A2") using zero kinetic energy photoelectron spectroscopy and coherent VUV radiation. The spectra about 8000 cm(-1) above the ground vibrational state of PH3+(X2A2") have been recorded. We observed the vibrational energy level splittings of PH3+(X2A2") due to the tunneling effect in the inversion (symmetric bending) vibration (nu2+). The energy splitting for the first inversion vibrational state (0+/0-) is 5.8 cm(-1). The inversion vibrational energy levels, rotational constants, and adiabatic ionization energies (IEs) for nu2+ = 0-16 have been determined. The bond angles between the neighboring P-H bonds and the P-H bond lengths are also obtained using the experimentally determined rotational constants. With the increasing of the inversion vibrational excitations (nu2+), the bond lengths (P-H) increase a little and the bond angles (H-P-H) decrease a lot. The inversion vibrational energy levels have also been calculated by using one dimensional potential model and the results are in good agreement with the experimental data for the first several vibrational levels. In addition to inversion vibration, we also observed firstly the other two vibrational modes: the symmetric P-H stretching vibration (nu1+) and the degenerate bending vibration (nu4+). The fundamental frequencies for nu1+ and nu4+ are 2461.6 (+/-2) and 1043.9 (+/-2) cm(-1), respectively. The first IE for PH3 was determined as 79670.9 (+/-1) cm(-1).     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

The conversion of phenylpropanedinitrile (phenylmalononitrile) into the carbanion, followed by IR spectra, ab initio and DFT force field calculations

The spectral and structural changes, caused by the conversion of phenylpropanedinitrile (phenylmalononitrile) into the carbanion, have been followed by IR spectra, ab initio HF, MP2 and DFT BLYP force field calculations. In agreement between theory and experiment, the conversion is accompanied with strong frequency decreases (with 114 cm(-1), mean value) of the cyano stretching bands nu(C triple bond N), dramatic increases in the corresponding integrated intensities (136-fold, total value), strong enhancement of the nu(C triple bond N) vibrational coupling and other essential spectral changes. According to the calculations, the strongest structural changes take place at the carbanionic center: (i) shortenings of the Cz-Ph and Cz-CN bonds with 0.064-0.092 A, and increases in the corresponding bond orders with 0.14-0.21 U; (ii) simultaneous enlargements of the bond angles at the same carbon atom with 7.6 degrees -9.7 degrees, as from tetrahedral its configuration becomes trigonal. The carbanionic charge is distributed between the two cyano groups (0.44-0.52 e(-)), phenyl ring (0.31-0.34 e(-)) and carbanionic center (0.14-0.25 e(-)). The formation of moderately strong (CH(3))(2)S=O...H-C(CN)(2)C(6)H(5) hydrogen bonds has been found experimentally.     

High-resolution absorption spectrum of jet-cooled CH3Cl between 70,000 and 85,000 cm(-1): new assignments

The absorption spectrum of jet-cooled CH(3)Cl was photographed from 165 to 117 nm (or 60,000 - 85,000 cm(-1), 7.5-10.5 eV) at a resolution limit of 0.0008 nm (0.3-0.6 cm(-1) or 0.04-0.08 meV). Even in the best structured region of the spectrum, from 70,000 to 85,000 cm(-1) (8.7-10.5 eV), observed bandwidths (full width at half maximum) are large, from 50 to 150 cm(-1). No rotational feature could be resolved. The spectrum is dominated by two strong bands near 9 eV, 140 nm, the D and E bands of Mulliken [J. Chem. Phys. 8, 382 (1940)] or the spectral region D of Price [J. Chem. Phys.4, 539 (1936)]. Their relative intensity is incompatible with previous assignments, namely, to a triplet and a singlet state belonging to the same configuration. On the basis of the present ab initio calculations, those bands are now assigned to two singlet states, the (1)A(1) and (1)E excited states resulting from the 2e(3)4pe Rydberg configuration. The present calculations also reveal that the two (1)E states issued from 2e(3)4sa(1) and 2e(3)4pa(1) are quasidegenerate and strongly mixed. They should be assigned to the two broad bands near 8 eV, 160 nm, the B and C bands of Mulliken and Price. Three vibrational modes are observed to be active: the CCl bond stretch nu(3)(a(1)), and the CH(3) umbrella and rocking vibrations, respectively, nu(2)(a(1)) and nu(6)(e). The fundamental frequencies deduced are well within the ranges defined by the corresponding values in the neutral and ion ground states. The possibility of a dynamical Jahn-Teller effect induced by the nu(6)(e) vibrational mode in the (1)E Rydberg states is discussed.     

Dihydrogen complexes of iridium and rhodium

A series of iridium and rhodium pincer complexes have been synthesized and characterized: [(POCOP)Ir(H)(H(2))] [BAr(f)(4)] (1-H(3)), (POCOP)Rh(H(2)) (2-H(2)), [(PONOP)Ir(C(2)H(4))] [BAr(f)(4)] (3-C(2)H(4)), [(PONOP)Ir(H)(2))] [BAr(f)(4)] (3-H(2)), [(PONOP)Rh(C(2)H(4))] [BAr(f)(4)] (4-C(2)H(4)) and [(PONOP)Rh(H(2))] [BAr(f)(4)] (4-H(2)) (POCOP = κ(3)-C(6)H(3)-2,6-[OP(tBu)(2)](2); PONOP = 2,6-(tBu(2)PO)(2)C(5)H(3)N; BAr(f)(4) = tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydrogen-metal interaction was probed using NMR spectroscopic studies. Complexes 1-H(3), 2-H(2), and 4-H(2) retain the H-H bond and are classified as η(2)-dihydrogen adducts. In contrast, complex 3-H(2) is best described as a classical dihydride system. The presence of bound dihydrogen was determined using both T(1) and (1)J(HD) coupling values: T(1) = 14 ms, (1)J(HD) = 33 Hz for the dihydrogen ligand in 1-H(3), T(1)(min) = 23 ms, (1)J(HD) = 32 Hz for 2-H(2), T(1)(min) = 873 ms for 3-H(2), T(1)(min) = 33 ms, (1)J(HD) = 30.1 Hz for 4-H(2).     

Thermochemistry of the fluoroformyloxyl radical: a computational study based on coupled cluster theory

The standard enthalpy of formation of FCO(2) (X (2)B(2)) was determined by a computational approach based on coupled cluster theory [CCSD(T)] with energies extrapolated to the basis-set limit, with additional corrections accounting for core-valence correlation, scalar relativity, spin-orbit coupling, and zero-point vibrational motions. Utilizing a variety of independent reaction schemes, our best estimate is Delta(f)H(o)(0)(FCO(2)) = -86.0 +/- 0.6 kcal mol(-1) [Delta(f)H(o)(298) )(FCO(2)) = -86.7 +/- 0.6 kcal mol(-1)], which is shown to be more accurate than previous theoretical and experimental values. The chosen computational procedure was also applied to HCO (X (2)A'), where we find excellent agreement with experiment, and to FCO (X (2)A'), where we recommend an improved value of Delta(f)H(o)(0)(FCO) = -42.1 +/- 0.5 kcal mol(-1) [ Delta(f)H(o)(298)(FCO) = -42.0 +/- 0.5 kcal mol(-1)]. Further theoretical results concern the C-F bond dissociation energy, electron affinity, ionization energy, first and second excitation energies in FCO(2), fluoride ion affinity of CO(2), and equilibrium geometries of the molecules treated presently. For FCO (X (2)A') we propose an improved equilibrium structure: r(e)(CF) = 132.5(2) pm, r(e)(CO) = 116.7(2) pm, and theta(e)(FCO) = 127.8(2)(o).     

Fourier transform infrared observation and density functional theory study of the AlC3 and AlC3Al linear chains trapped in solid Ar

The vibrational spectra of linear AlC(3) and AlC(3)Al, formed by trapping the products of the dual laser evaporation of aluminum and carbon rods in solid Ar at approximately 10 K, were observed. Fourier transform infrared (FTIR) measurements of (13)C isotopic shifts are in good agreement with the predictions of density functional theory (DFT) B3LYP6-311+G(3df) calculations, enabling the first assignments of the nu(3)(sigma(u)) and nu(4)(sigma(u)) fundamentals of ((3)Sigma(g) (+)) linear AlC(3)Al at 1624.0 and 528.3 cm(-1), respectively, and the nu(2)(sigma) vibrational fundamental of ((2)Pi) linear AlC(3) at 1210.9 cm(-1).     

The photodissociation reaction dynamics of CF(3)I at 304 nm ((3)Q(0+), (1)Q(1)<--(3)Q(0+), and (3)Q(1))

The photodissociation of CF(3)I at 304 nm has been studied using long time-delayed core-sampling photofragment translational spectroscopy. Due to its capability of detecting the kinetic energy distribution of iodine fragments with high resolution, it is able to directly assign the vibrational state distribution of CF(3) fragments. The vibrational state distributions of CF(3) fragments in the I(*)((2)P(12)) channel, i.e., (3)Q(0+) state, have a propensity of the nu(2) (') umbrella mode with a maximum distribution at the vibrational ground state. For the I((2)P(32)) channel, i.e., (1)Q(1)<--(3)Q(0+), the excitation of the nu(2) (') umbrella mode accounts for the majority of the vibrational excitation of the CF(3) fragments. The 1 nu(1) (') (symmetric CF stretch) +nnu(2) (') combination modes, which are associated with the major progression of the nu(2) (') umbrella mode, are observed for the photodissociation of CF(3)I at the I channel, i.e., (3)Q(1) state. The bond dissociation energy of the CI bond of CF(3)I is determined to be D(0)(CF(3)-I)相似文献   

In search of definitive signatures of the elusive NCCO radical

Previous experimental assignments of the fundamental vibrational frequencies of NCCO have been brought into question by subsequent unsuccessful attempts to observe IR signatures of this radical at these frequencies. Here we compute the fundamental vibrational frequencies by applying second-order vibrational perturbation theory to the complete quartic force field computed at the all-electron (AE) coupled cluster singles, doubles, and perturbative triples level [CCSD(T)] with the correlation-consistent, polarized core-valence quadruple-zeta (cc-pCVQZ) basis set, which has tight functions to correctly describe core correlation. The AE-CCSD(T)/cc-pCVQZ geometric parameters are r(e)(N-C)=1.1623 A, r(e)(C-C)=1.4370 A, r(e)(C-O)=1.1758 A, theta(e)(N-C-C)=168.55 degrees , and theta(e)(C-C-O)=132.22 degrees . Our CCSD(T)/cc-pCVQZ values of the characteristic stretching frequencies nu(1) and nu(2) are 2171 and 1898 cm(-1), respectively, in stark contrast to the experimentally derived values of 2093 and 1774 cm(-1). Finally, focal-point extrapolations using correlation-consistent basis sets cc-pVXZ (X=D,T,Q,5,6) and electron correlation treatments as extensive as full coupled cluster singles, doubles, and triples (CCSDT) with perturbative accounting of quadruple excitations [CCSDT(Q)] determine the vibrationless barrier to linearity of NCCO and the dissociation energy (D(0)) of NCCO-->NC+CO to be 8.4 and 26.5 kcal mol(-1), respectively. Using our precisely determined dissociation energy, we recommend a new 0 K enthalpy of formation for NCCO of 50.9+/-0.3 kcal mol(-1).     

The diazocarbene (CNN) molecule: characterization of the X 3Sigma- and A 3Pi electronic states

The ground (X (3)Sigma(-)) and first excited triplet (A (3)Pi) electronic states of diazocarbene (CNN) have been investigated systematically starting from the self-consistent-field theory and proceeding to the coupled cluster with single, double, and full triple excitations (CCSDT) method with a wide range of basis sets. While the linear X (3)Sigma(-) ground state of CNN has a real degenerate bending vibrational frequency, the A (3)Pi state of CNN is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequencies of the A (3)Pi state were evaluated via the equation-of-motion coupled cluster (EOM-CC) techniques. The significant sensitivity to level of theory in predicting the ground-state geometry, harmonic vibrational frequencies, and associated infrared intensities has been attributed to the fact that the reference wave function is strongly perturbed by the excitations of 1pi-->3pi followed by a spin flip. At the highest level of theory with the largest basis set, correlation-consistent polarized valence quadruple zeta (cc-pVQZ) CCSDT, the classical X-A splitting (T(e) value) was predicted to be 68.5 kcal/mol (2.97 eV, 24 000 cm(-1)) and the quantum mechanical splitting (T(0) value) to be 69.7 kcal/mol (3.02 eV, 24 400 cm(-1)), which are in excellent agreement with the experimental T(0) values, 67.5-68.2 kcal/mol (2.93-2.96 eV, 23 600-23 900 cm(-1)). With the EOM-CCSD method the Renner parameter (epsilon) and averaged bending vibrational frequency (omega(2)) for the A (3)Pi state were evaluated to be epsilon=-0.118 and omega(2)=615 cm(-1), respectively. They are in fair agreement with the experimental values of epsilon=-0.07 and nu(2)=525 cm(-1).