Optical spectroscopy of RuC: 18,000-24,000 cm(-1)

The optical spectrum of diatomic RuC has been recorded from 17 800 to 24 200 cm(-1). Three previously unidentified excited electronic states were analyzed and identified as having Omega' = 0, Omega' = 2, and Omega' = 3. The Omega' = 3 state was determined to be a 3Delta3 state that is suggested to arise from a mixture of the 10sigma(2)11sigma(2)5pi(3)2delta(3)12sigma(1)6pi(1) and 10sigma(2)11sigma(1)5pi(3)2delta(3)12sigma(2)6pi(1) electronic configurations. Three additional bands belonging to the previously observed [18.1] (1)Pi<--X (1)Sigma(+) system were analyzed to obtain B(e) (')=0.558 244(48) cm(-1), alpha(e) (')=0.004 655(27) cm(-1), omegae' = 887.201(37) cm(-1), and omega(e) 'xe' = 5.589(7) cm(-1) for the 102Ru 12C isotopomer (1sigma error limits). A Rydberg-Klein-Rees analysis was then performed using the determined spectroscopic constants of the [18.1] 1Pi state, and similar analyses were performed for the previously observed states. The resulting potential energy curves are provided for the 100Ru 12C, 101Ru 12C, 102Ru 12C, and 104Ru 12C isotopic species.     

Resonant two-photon ionization spectroscopy of jet-cooled OsC

The optical spectrum of diatomic OsC has been investigated for the first time, with transitions recorded in the range from 17 390 to 22 990 cm(-1). Six bands were rotationally resolved and analyzed to obtain ground and excited state rotational constants and bond lengths. Spectra for six OsC isotopomers, 192 Os 12C (40.3% natural abundance), 190 Os 12C(26.0%), 189 Os 12C(16.0%), 188 Os 12C(13.1%), 187 Os 12C(1.9%), and 186 Os 12C(1.6%), were recorded and rotationally analyzed. The ground state was found to be X 3 Delta 3, deriving from the 4 delta 3 16 sigma 1 electronic configuration. Four bands were found to originate from the X 3 Delta 3 ground state, giving B 0"=0.533 492(33) cm(-1) and r 0 "=1.672 67(5) A for the 192 Os 12C isotopomer (1 sigma error limits); two of these, the 0-0[19.1]2<--X 3 Delta 3 and 1-0[19.1]2<--X 3 Delta 3 bands, form a vibrational progression with Delta G' 1/2=953.019 cm(-1). The remaining two bands were identified as originating from an Omega"=0 level that remains populated in the supersonic expansion. This level is assigned as the low-lying A 3 Sigma 0+ (-) state, which derives from the 4 delta 2 16 sigma 2 electronic configuration. The OsC molecule differs from the isovalent RuC molecule in having an X 3 Delta 3 ground state, rather than the X 2 delta 4, 1 Sigma+ ground state found in RuC. This difference in electronic structure is due to the relativistic stabilization of the 6s orbital in Os, an effect which favors occupation of the 6s-like 16 sigma orbital. The relativistic stabilization of the 16 sigma orbital also lowers the energy of the 4 delta 2 16 sigma 2, 3 Sigma(-) term, allowing this term to remain populated in the supersonically cooled molecular beam.     

Multireference configuration interaction study of the electronic states of ZrC

The potential energy curves and spectroscopic constants of the ground and 32 low-lying electronic states of ZrC have been studied by employing multireference configuration interaction methods, in conjunction with relativistic effective core potentials and 5s3p3d1f, 3s3p1d basis sets con Zr and C, respectively. We have determined that the ground state is (3)Sigma(+). However there are two low-lying (1)Sigma(+) states (below 5000 cm(-1)) which strongly interact resulting in avoided crossings. The lowest (1)Sigma(+) state corresponds to a combination of 1sigma(2) Xsigma(2) 1pi(4) configurations whereas the second is an open shell singlet 1sigma(2) 2sigma(1) 3sigma(1) 1pi(4). Several avoided crossings were observed, for (1)Pi, (3)Pi, (1)Delta, (3)Sigma(+), and (3)Delta states. We have identified (3)Pi and (1)Pi lying at 4367 and 5797 cm(-1), respectively. The results are in good agreement with the recent experimental findings of Rixon et al. [J. Mol. Spectrosc. 228, 554 (2004)], and indicate that the (3)Pi-(3)Sigma(+), and (1)Pi-(1)Sigma(+), bands located between 16 000-19 000 cm(-1) are extremely complex due to near degeneracy of several (1)Pi and (3)Pi states. We also have identified a (1)Sigma(+) state in the same region that may interfere with the (1)Pi emission bands. The present results not only shed further light into the spectra of ZrC but also predict yet to be observed systems.     

1 Pi<--X1 Sigma+ band systems of jet-cooled ScCo and YCo

Rotationally resolved resonant two-photon ionization (R2PI) spectra of ScCo and YCo are reported. The measured spectra reveal that these molecules possess ground electronic states of (1)Sigma(+) symmetry, as previously found in the isoelectronic Cr(2) and CrMo molecules. The ground state rotational constants for ScCo and YCo are B(0)(")=0.201 31(22) cm(-1) and B(0) (")=0.120 96(10) cm(-1), corresponding to ground state bond lengths of r(0) (")=1.812 1(10) A and r(0) (")=1.983 0(8) A, respectively. A single electronic band system, assigned as a (1)Pi<--X (1)Sigma(+) transition, has been identified in both molecules. In ScCo, the (1)Pi state is characterized by T(0)=15,428.8, omega(e)(')=246.7, and omega(e)(')x(e)(')=0.73 cm(-1). In YCo, the (1)Pi state has T(0)=13 951.3, omega(e)(')=231.3, and omega(e)(')x(e) (')=2.27 cm(-1). For YCo, hot bands originating from levels up to v(")=3 are observed, allowing the ground state vibrational constants omega(e)(")=369.8, omega(e)(")x(e)(")=1.47, and Delta G(12)(")=365.7 cm(-1) to be deduced. The bond energy of ScCo has been measured as 2.45 eV from the onset of predissociation in a congested vibronic spectrum. A comparison of the chemical bonding in these molecules to related molecules is presented.     

Solvent effects on the electrochemistry and spectroelectrochemistry of diruthenium complexes. Studies of Ru2(L)4Cl where L=2-CH3ap, 2-Fap, and 2,4,6-F3ap, and ap is the 2-anilinopyridinate anion

Three Ru2(5+) diruthenium complexes, (4,0) Ru2(2-CH3ap)4Cl, (3,1) Ru2(2-Fap)4Cl, and (3,1) Ru2(2,4,6-F3ap)4Cl where ap is the 2-anilinopyridinate anion, were examined as to their electrochemical and spectroelectrochemical properties in five different nonaqueous solvents (CH2Cl2, THF, PhCN, DMF, and DMSO). Each compound undergoes a single one-electron metal-centered oxidation in THF, DMF, and DMSO and two one-electron metal-centered oxidations in CH2Cl2 and PhCN. The three diruthenium complexes also undergo two reductions in each solvent except for CH2Cl2, and these electrode processes are assigned as Ru2(5+/4+) and Ru2(4+/3+). Each neutral, singly reduced, and singly oxidized species was characterized by UV-vis thin-layer spectroelectrochemistry, and the data are discussed in terms of the most probable electronic configuration of the compound in solution. The three neutral complexes contain three unpaired electrons as indicated by magnetic susceptibility measurements using the Evans method (3.91-3.95 muB), and the electronic configuration is assigned as sigma2pi4delta2pi(*2)delta, independent of the solvent. The three singly oxidized compounds have two unpaired electrons in CD2Cl2, DMSO-d6, or CD3CN (2.65-3.03 muB), and the electronic configuration is here assigned as sigma2pi4delta2pi(*2). The singly reduced compound also has two unpaired electrons (2.70-2.80 muB) in all three solvents, consistent with the electronic configuration sigma2pi4delta2pi(*2)delta(*2) or sigma2pi4delta2pi(*3)delta*. Finally, the overall effect of solvent on the number of observed redox processes is discussed in terms of solvent binding, and several formation constants were calculated.     

A full analytic potential energy curve for the a 3 Sigma+ state of KLi from a limited vibrational data set

Fourier transform spectra of near-infrared laser-induced fluorescence in (39)K(6)Li show transitions to high vibrational levels of both the X (1)Sigma(+) and a (3)Sigma(+) electronic states. These include 147 transitions into six vibrational levels of the a (3)Sigma(+) state, which lie between 7 and 88 cm(-1) below the dissociation asymptote. Unfortunately, their energies span less than 30% of the well depth. However, fitting those data to eigenvalues of analytical model potential functions whose outer limbs incorporate the theoretically predicted long-range form, V(R) approximately D-C(6)R(6)-C(8)R(8), yields complete, plausible potential curves for this state. The best fits converge to remarkably similar solutions which indicate that D(e)=287(+/-4) cm(-1) and R(e)=4.99(+/-0.09) A for the a (3)Sigma(+) state of KLi, with omega(e)=47.3(+/-1.4) and 44.2(+/-1.5) cm(-1) for (39)K(6)Li and (39)K(7)Li, respectively. Properties of the resulting potential are similar to those of a published ab initio potential and are consistent with those of the analogous states of Li(2), K(2), Na(2), and NaK.     

The singlet electronic ground state isomers of dialuminum monoxide: AlOAl, AlAlO, and the transition state connecting them

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol(-1) below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-1), presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl-->AlAlO and AlAlO-->AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.     

Low-lying electronic states of FeNC and FeCN: a theoretical journey into isomerization and quartet/sextet competition

With several levels of multireference and restricted open-shell single-reference electronic structure theory, optimum structures, relative energetics, and spectroscopic properties of the low-lying (6)Delta, (6)Pi, (4)Delta, (4)Pi, and (4)Sigma(-) states of linear FeNC and FeCN have been investigated using five contracted Gaussian basis sets ranging from Fe[10s8p3d], C/N[4s2p1d] to Fe[6s8p6d3f2g1h], C/N[6s5p4d3f2g]. Based on multireference configuration interaction (MRCISD+Q) results with a correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis set, appended with core correlation and relativistic corrections, we propose the relative energies: T(e)(FeNC), (6)Delta(0)<(6)Pi (2300 cm(-1))<(4)Delta (2700 cm(-1))<(4)Pi (4200 cm(-1))<(4)Sigma(-); and T(e)(FeCN), (6)Delta(0)<(6)Pi (1800 cm(-1))<(4)Delta (2500 cm(-1))<(4)Pi (2900 cm(-1))<(4)Sigma(-). The (4)Delta and (4)Pi states have massive multireference character, arising mostly from 11sigma-->12sigma promotions, whereas the sextet states are dominated by single electronic configurations. The single-reference CCSDT-3 (coupled cluster singles and doubles with iterative partial triples) method appears to significantly overshoot the stabilization of the quartet states provided by both static and dynamical correlation. The (4,6)Delta and (4,6)Pi states of both isomers are rather ionic, and all have dipole moments near 5 D. On the ground (6)Delta surface, FeNC is predicted to lie 0.6 kcal mol(-1) below FeCN, and the classical barrier for isocyanide/cyanide isomerization is about 6.5 kcal mol(-1). Our data support the recent spectroscopic characterization by Lei and Dagdigian [J. Chem. Phys. 114, 2137 (2000)] of linear (6)Delta FeNC as the first experimentally observed transition-metal monoisocyanide. Their assignments for the ground term symbol, isotopomeric rotational constants, and the Fe-N omega(3) stretching frequency are confirmed; however, we find rather different structural parameters for (6)Delta FeNC:r(e)(Fe-N)=1.940 A and r(N-C)=1.182 A at the cc-pVQZ MRCISD+Q level. Our results also reveal that the observed band of FeNC originating at 27 236 cm(-1) should have an analog in FeCN near 23 800 cm(-1) of almost equal intensity. Therefore, both thermodynamic stability and absorption intensity factors favor the eventual observation of FeCN via a (6)Pi<--(6)Delta transition in the near-UV.     

The low-lying electronic states of nickel cyanide and isocyanide: A theoretical investigation

At different levels of coupled cluster theory optimum structures, energetics, and harmonic vibrational frequencies for several low-lying doublet and quartet electronic states of linear NiCN and NiNC were studied using four contracted Gaussian basis sets, ranging from Ni[6s5p4d2f], CN[4s3p2d] to Ni[8s7p5d3f2g1h], CN[5s4p3d2f1g]. The most reliable predictions were obtained with a relativistic Douglas-Kroll restricted open-shell-based coupled cluster method including singles, doubles, and perturbative triple excitations [DK-R/UCCSD(T)]. This level of theory was used in conjunction with correlation-consistent polarized valence Douglas-Kroll recontracted quadruple-zeta basis sets (cc-pVQZDK). The energetic ordering of the electronic states of NiCN is predicted to be 2delta < 2sigma+ < 2pi < 4delta < 4pi and that of NiNC is 2delta approximately 2sigma+ < 2pi < 4delta < 4pi < 4sigma-. Our theoretical investigation supports the assignment of the ground-state term symbol, the Ni-C stretching frequency, and the bending frequency for the ground electronic state of NiCN by Kingston et al. [J. Mol. Spectrosc. 215, 106 (2002)] and by Sheridan and Ziurys [J. Chem. Phys. 118, 6370 (2003)]. The predicted structure of the 2delta ground state of NiCN, r(e)(Ni-C) = 1.822 angstroms and r(e)(C-N) = 1.167 angstroms, at DK-R/UCCSD(T)/cc-pVQZDK shows excellent agreement with the experimentally determined Ni-C bond length of 1.826 A and less satisfactory agreement for the C-N bond length of 1.153 angstroms [J. Chem. Phys. 118, 6370 (2003)]. It is also concluded that the metal-to-ligand pi back donation is weak or negligible. Additionally, we found that on the 2delta surface the linear cyanide isomer lies lower in energy than the linear isocyanide isomer by 12.2 kcal mol(-1).     

Ab initio study of the mechanism for photoinduced Yl-oxygen exchange in uranyl(VI) in acidic aqueous solution

The mechanism for the photochemically induced isotope-exchange reaction U(17/18)O2(2+)(aq) + H2(16)O <==> U(16)O2(2+)(aq) + H2(17/18)O has been studied using quantum-chemical methods. There is a dense manifold of states between 22,000 and 54,000 cm(-1) that results from excitations from the sigma(u) and pi(u) bonding orbitals in the (1)Sigma(g)(+) ground state to the nonbonding f(delta) and f(phi) orbitals localized on uranium. On the basis of investigations of the reaction profile in the (1)Sigma(g)(+) ground state and the excited states (3)Delta(g) (the lowest triplet state) and (3)Gamma(g) (one of the several higher triplet states), the latter two of which have the electron configurations sigma(u)f(delta) and pi(u)f(phi), respectively, we suggest that the isotope exchange takes place in one of the higher triplet states, of which the (3)Gamma(g) state was used as a representative. The geometries of the luminescent (3)Delta(g) state, the lowest in the sigma(u)f(delta,phi) manifold (the "sigma" states), and the (1)Sigma(g)(+) ground state are very similar, except that the bond distances are slightly longer in the former. This is presumably a result of transfer of a bonding electron to a nonbonding f orbital, which makes the excited state in some respects similar to uranyl(V). As is the case for all of the states of the pi(u)f(delta,phi) manifold (the "pi" states), the geometry of the (3)Gamma(g) state is very different from that of the (3)Delta(g) "sigma" state and has nonequivalent U-O(yl) distances of 1.982 and 1.763 A; in the (3)Gamma(g) state, the yl-exchange takes place by transfer of a proton or hydrogen from water to the more distant yl-oxygen. The activation barriers for proton/hydrogen transfer in the ground state and the (3)Delta(g) and (3)Gamma(g) states are 186, 219, and 84 kJ/mol, respectively. The relaxation energy for the (3)Gamma(g) state in the solvent after photoexcitation is -86 kJ/mol, indicating that the energy barrier can be overcome; the "pi" states are therefore the most probable route for proton/hydrogen transfer. They can be populated after UV irradiation but are too high in energy (approximately 36,000-40,000 cm(-1)) to be reached by a single-photon absorption at 436 nm (22,900 cm(-1)), where experimental data have demonstrated that exchange can take place. Okuyama et al. [Bull. Res. Lab. Nucl. React. (Tokyo Inst. Technol.) 1978, 3, 39-50] have demonstrated that an intermediate is formed when an acidic solution of UO2(2+)(aq) is flash-photolyzed in the UV range. The absorption spectrum of this short-lived intermediate (which has a maximum at 560 nm) indicates that this species arises from 436 nm excitation of the luminescent (3)Delta(g) state (which has a lifetime of approximately 2 x 10(-6) s); this is sufficient to reach the reactive "pi" states. It has been speculated that the primary reaction in acidic solutions of UO2(2+)(aq) is the formation of a uranyl(V) species; our results indicate that the structure in the luminescent state has some similarity to that of UO2(+) but that the reactive species in the "pi" states is a cation radical with a distinctly different structure.     

First spectroscopic studies of the B2Sigma+ --> X2Sigma+ system in the 12C17O+ molecule spectrum

The emission spectrum of the B2Sigma+ --> X2Sigma+ system in the 12C17O+ has been recorded at high resolution with conventional spectroscopic techniques. A graphite hollow cathode lamp filled with oxygen, enriched approximately 45% with the 17O2 isotope, has given strong spectra of hitherto unreported bands system of 12C17O+. Four bands, which form the 0-v(') (v(')=1 -4) progression, have been rotationally analysed with effective Hamiltonians of Brown [J. Mol. Spectrosc. 74 (1979) 294]. Next the rovibronic structure parameters, for the B2Sigma+ and X2Sigma+ states, have been determined from merge calculations. Finally, principal equilibrium molecular constants for the ground state: omegae=2186.032(26) cm(-1), omegae xe=14.7848(43) cm(-1), Be=1.927271(73) cm(-1) , alphae=1.8432(24) x 10(-2) cm(-1), De=6.018(26) x 10(-6) cm(-1), betae=1.11(92) x 10(-8) cm(-1) and the sigma(e)(B-->X)=45876.563(34) cm(-1) value of the 12CO17+ molecule have been derived for the first time in this investigation.     

Study on the photodissociation spectra of CS2+ via B2Sigma u+ and C2Sigma g+ electronic states

The photodissociation spectra of CS(2)(+) ions via B(2)Sigma(u)(+) and C(2)Sigma(g)(+) electronic states have been studied by using two-photon excitation, where the parent CS(2)(+) ions were prepared by [3 + 1] REMPI (resonance-enhanced multiphoton ionization) at 483.2 nm from the jet-cooled CS(2) molecules. The [1 + 1] photodissociation spectrum of CS(2)(+) via the B(2)Sigma(u)(+)(upsilon(1)upsilon(2)0) <-- X(2)Pi(g,3/2)(000) transition was obtained by scanning the dissociation laser in the wavelength range of 270-285 nm and detecting the signal of both S(+) and CS(+). The [1 + 1'] photodissociation spectra of CS(2)(+) were obtained by fixing the first dissociation laser at 281.94 or 277.15 nm to excite the B(2)Sigma(u)(+) (000 or 100) <-- X(2)Pi(g,3/2)(000) transitions and scanning the second dissociation laser in the range of 606-763 nm to excite C(2)Sigma(g)(+)(upsilon(1)upsilon(2)0) <-- B(2)Sigma(u)(+)(000,100) transitions. New spectroscopic constants of nu(1) = 666.2 +/- 2.5 cm(-1), nu(2) = 363.2 +/- 1.9 cm(-1), chi(11) = -5.5 +/- 0.1 cm(-1), chi(22) = 1.6 +/- 0.1 cm(-1), chi(12) = -8.6 +/- 0.2 cm(-1), and k(122) = 44.9 +/- 2.5 cm(-1) (Fermi resonance constant) for the C(2)Sigma(g)(+) state are deduced from the [1 + 1'] photodissociation spectra. On the basis of the [1 + 1] and [1 + 1'] photodissociation spectra, the wavelength and level dependence of the product branching ratios CS(+)/S(+) has been found and the dissociation dynamics of CS(2)(+) ions via B(2)Sigma(u)(+) and C(2)Sigma(g)(+) electronic states are discussed.     

Metal-metal bonding in molecular actinide compounds: electronic structure of [M2X8](2-) (M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues

Density functional and multiconfigurational (ab initio) calculations have been performed on [M(2)X(8)](2-) (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Multiple metal-metal bonds consisting of a combination of sigma and pi interactions have been found in all species investigated, with delta-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal-metal interactions possess predominantly d(z2) (sigma), d(xz) and d(yz) (pi), or d(xy) and d(x2-y2) (delta) character in the d-block species, and f(z3) (sigma), f(z2x) and f(z2y) (pi), or f(xyz) and f(z) (delta) character in the actinide systems. In the latter, all three (sigma, pi, delta) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D(4h) or D(4d). By contrast, although the nature and properties of the sigma and pi bonds are largely similar for the D(4h) and D(4d) forms of the d-block complexes, the two most relevant metal-metal delta-like orbitals occur as a bonding and antibonding combination in D(4h) symmetry but as a nonbonding level in D(4d) symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.     

Theoretical study of electronic states of N22+in an intense radiation field

The Floquet states of N(2) (2+) created by the interactions of the six lowest singlet (1 (1)Sigma(g) (+), 1 (1)Delta(g), 2 (1)Sigma(g) (+), 1 (1)Pi(u), 1 (1)Pi(g), and 1 (1)Sigma(u) (-)) states of the dication with intense (0.4 x 10(13) Wcm(2)) radiation have been studied using the recently developed multireference configuration interaction method with single and double excitations (MRCISD)-based approach. The adiabatic Floquet state coinciding near its minimum with the initial X (1)Sigma(g) (+) ground state and asymptotically correlating with A (1)Pi(u) (m = -1), i.e., with one less photon in the dressed state, is expected to be metastable, as is the ground state in the absence of a field, but to support up to the v(max) = 12 quasibound vibrational level in comparison with v(max) = 11 in the parent field-free X (1)Sigma(g) (+) ground state. The tunneling lifetimes of the highest vibrational levels in this adiabatic Floquet state are predicted to be several orders longer than those in the parent field-free state. Analysis of the complete basis set limit extrapolated MRCISD potential energy curve of the field-free X (1)Sigma(g) (+) state of N(2) (2+) calculated in the present work (R(e) = 1.130 A, omega(e) = 2011 cm(-1), omega(e)x(e) = 26.1 cm(-1)) is in good agreement with spectroscopic experimental data. Calculations on the field-free A (1)Pi(u) state (T(e) = 12 106 cm(-1), R(e) = 1.252 A, omega(e) = 1438 cm(-1), omega(e)x(e) = 23.5 cm(-1)) generally support earlier theoretical work and do not support reported experimental values.     

Preparation,structure, and unique thermal [2+2], [4+2], and [3+2] cycloaddition reactions of 4-vinylideneoxazolidin-2-one

The terminal allene C(alpha)=C(beta) bonds of 4vinylidene-2-oxazolidinone (2) readily undergo [2+2] cycloaddition with a wide variety of terminal alkynes, alkenes, and 1,3-dienes irrespective of their electronic nature under strictly thermal activation conditions (70-100 degrees C) and provide 3substituted (Z)-methylenecyclobutenes 6, 3substituted methylenecyclobutanes 7 and 8, and 3vinylmethylenecyclobutanes 9, respectively, in good to excellent yields. Alkenes react with 2 with complete retention of configuration. The [2+2] cycloaddition is concluded to proceed via a concerted [(pi(2s)+pi(2s))(allene) + pi(2s)] Hückel transition state on the basis of experimental evidences and quantum mechanical methods. Some highly polarized enones and nitrile oxide, on the other hand, react with 2 selectively at the internal C(4)=C(alpha) double bonds and give spiro compounds 10 and 11, respectively. The bent allene bonds (173-176 degrees) and the unique reactivity associated with 2 are attributed to a low-lying LUMO (C(alpha)=C(beta)) that is substantiated by a through-space sigma*(N-SO(2))-pi*(C(alpha)=C(beta)) orbital interaction.     

Monoligated monovalent Ni: the 3d(Ni)9 manifold of states of NiCu and comparison to the 3d9 States of AlNi, NiH, NiCl, and NiF

A dispersed fluorescence investigation of the low-lying electronic states of NiCu has allowed the observation of four out of the five states that derive from the 3d(Ni)9 3d(Cu)10 sigma2 manifold. Vibrational levels of the ground X2delta(5/2) state corresponding to v = 0-11 are observed and are fit to provide omega(e) = 275.93 +/- 1.06 cm(-1) and omega(e)x(e) = 1.44 +/- 0.11 cm(-1). The v = 0 levels of the higher lying states deriving from the 3d(Ni)9 3d(Cu)10 sigma2 manifold are located at 912, 1466, and 1734 cm(-1), and these states are assigned to omega values of 3/2, 1/2, and 3/2, respectively. The last of these assignments is based on selection rules and is unequivocal; the first two are based on a comparison to ab initio and ligand field calculations and could conceivably be in error. It is also possible that the v = 0 level of the state found at 912 cm(-1) is not observed, so that T0 for the lowest excited state actually lies near 658 cm(-1). These results are modeled using a matrix Hamiltonian based on the existence of a ground manifold of states deriving from the 3d9 configuration on nickel. This matrix Hamiltonian is also applied to the spectroscopically well-known molecules AlNi, NiH, NiCl, and NiF. The term energies of the 2sigma+, 2pi, and 2delta states of these molecules, which all derive from a 3d9 configuration on the nickel atom, display a clear and understandable trend as a function of the electronegativity of the ligands.     

Doubly excited 2 1Delta g state of Na2

The doubly excited valence (3p+3p) 2 (1)Delta(g) state of Na(2) is experimentally observed by using optical-optical double resonance spectroscopy. A single line Ar(+) laser (a total of nine lines) was used to pump the sodium dimers from thermally populated ground state X (1)Sigma(g) (+) to the intermediate B (1)Pi(u) state. Then, a single mode Ti:sapphire laser was used to probe the doubly excited 2 (1)Delta(g) state. Violet fluorescence emitted from the highly excited states (mainly 2 (3)Pi(g) or 3 (3)Pi(g) states which are transferred from 2 (1)Delta(g) state via collision) to the a (3)Sigma(u) (+) state was monitored by a filtered photomultiplier tube (PMT). A total of 582 rovibrational levels of 2 (1)Delta(g) state were observed, identified, and assigned to the vibrational and rotational quantum numbers in the range of 0< or =v< or =28 and 11< or =J< or =99, respectively. The absolute vibrational quantum number assignment was verified by comparing the totally resolved fluorescence with the calculated Franck-Condon factors between 2 (1)Delta(g) state and B (1)Pi(u) state. Dunham coefficients and Rydberg-Klein-Rees potential curve were derived from these observed quantum levels. The primary molecular constants of Na(2) 2 (1)Delta(g) state are T(e)=32 416.759(15) cm(-1), omega(e)=124.8484(36) cm(-1), B(e)=0.119 158(3) cm(-1), and R(e)=3.508 20(5) A.     

Population transfer to excited vibrational levels of H2 molecule by stimulated hyper-Raman passage with chirped laser pulses

We have theoretically investigated the population transfer from the initial ground rovibrational level v(g)=0, J(g)=0 to the final rovibrational levels v(f)=1,2, J(f)=0 of the ground electronic state X (1)Sigma(g) (+) via the resonant intermediate level v(i)=6, J(i)=0 of the excited electronic state EF (1)Sigma(g) (+) of H(2) molecule by (2+2)-photon stimulated hyper-Raman passage (STIHRP). The density matrix technique has been employed to evaluate the population transfer to the final target levels using linearly chirped pump and Stokes laser pulses with different chirp rates. Both the pulses are considered to have the same temporal shape, pulse width, and linear parallel polarizations. We have studied in detail the dependence of the population transfer on the set of laser parameters for pulse (peak) intensities in the ranges of 1.5 x 10(11)-1.0 x 10(12) and 1.0 x 10(12)-7.0 x 10(12) W/cm(2). The corresponding pulse widths (full width at half maximum) are of the order of 115-200 and 15-30 ps. We have found that the chirp rate parameters can be optimized to achieve almost complete population transfer from the ground (g) to the final (f) target levels. This, to our knowledge, is the first application of a (2+2)-photon STIHRP process with chirpings to a model molecular system (H(2)). The study demonstrates the suitability of the chirped (2+2)-photon STIHRP technique for selective and almost total inversion of vibrational population in a diatomic molecule.     

The first observation of the rhodium monofluoride molecule: jet-cooled laser spectroscopic studies

Rhodium monofluoride has been observed and spectroscopically characterized. RhF molecules were produced under jet-cooled conditions in a laser vaporization molecular beam source by the reaction of a laser-vaporized rhodium plasma with SF(6) doped in helium, and studied with laser-induced fluorescence spectroscopy under both medium and high resolution. More than 25 bands have been observed in laser-induced fluorescence between 18,500 and 24,500 cm(-1) and five of these have been recorded at 200 MHz resolution. All bands of appreciable intensity have been rotationally analyzed. The ground electronic levels has Omega=2, which is attributed to an inverted (3)Pi state from the 2 delta(4)6 pi(3)12 sigma(1) electron configuration. The ground level rotational constants are B=0.272 45 cm(-1), D=1.035 x 10(-7) cm(-1). Very small ground level Lambda doublings are evident in the spectrum. Excited states having Omega=1, 2, and 3 have been identified. Dispersed fluorescence spectroscopy from 11 excited levels has been used to locate a large number of low-lying vibronic states within the energy range up to 8,000 cm(-1). A ground state vibrational interval of approximately 575 cm(-1) is suggested.     

The d (3)Pi(g)-c (3)Sigma(u) (+) band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).