Direct determination of the ionization energies of PtC, PtO, and PtO2 with VUV radiation

Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and IE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +/- 0.07 eV, D0(Pt-O) = 4.30 +/- 0.12 eV, and D0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules DeltaH(0)(f,0)(PtC(g)) = 701 +/- 7 kJ/mol, DeltaH(0)(f,0)(PtO(g)) = 396 +/- 12 kJ/mol, and DeltaH(0)(f,0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.     

Pulsed field-ionization photoelectron-photoion coincidence study of the process N2+hnu-->N++N+e-: bond dissociation energies of N2 and N2+

We have examined the dissociative photoionization reaction N2+hnu-->N++N+e- near its threshold using the pulsed field-ionization photoelectron-photoion coincidence (PFI-PEPICO) time-of-flight (TOF) method. By examining the kinetic-energy release based on the simulation of the N+ PFI-PEPICO TOF peak profile as a function of vacuum ultraviolet photon energy and by analyzing the breakdown curves of N+ and N2+, we have determined the 0-K threshold or appearance energy (AE) of this reaction to be 24.2884+/-0.0010 eV. Using this 0-K AE, together with known ionization energies of N and N2, results in more precise values for the 0-K bond dissociation energies of N-N (9.7543+/-0.0010 eV) and N-N+ (8.7076+/-0.0010 eV) and the 0-K heats of formation for N (112.469+/-0.012 kcal/mol) and N+ (447.634+/-0.012 kcal/mol).     

Thermochemical parameters of CHFO and CF2O

Thermochemical properties of CHFO and CF 2O and their derivatives were calculated by using coupled-cluster theory (U)CCSD(T) calculations with the aug-cc-pV nZ ( n = D, T, Q, 5) basis sets extrapolated to the complete basis set limit with additional corrections. The predicted properties include the following. Enthalpies of formation (298 K, kcal/mol): Delta H f (CF 2O) = -144.7, Delta H f(CHFO) = -91.1, Delta H f (CFO (*)) = -41.6. Bond dissociation energy (0 K, kcal/mol): BDE(CFO-F) = 120.7, BDE(CHO-F) = 119.1, BDE(CFO-H) = 100.2. Ionization potential (eV): IP 1(CF 2O) = 13.04, IP 2(CF 2O) = 14.09, IP 1(CHFO) = 12.41, IP 2(CHFO) = 13.99, IP 1(CFO (*)) = 9.34. Proton affinity (298 K, kcal/mol), PA O(CF 2O) = 148.8, PA O(CHFO) = 156.7, PA F(CHFO) = 154.5 kcal/mol. Electron affinity: EA(CFO (*)) = 2.38 eV. Triplet-singlet separation gap (eV): Delta E T1-S0(CF 2O) = 4.47, Delta E T1-S0(CHFO) = 4.36. Triplet-triplet transition energy (eV): Delta E T2-T1(CF 2O) = 0.44. The new calculated values contribute to solving some persistent discrepancies in the literature. The effects of F-atoms on thermochemical parameters are not linearly additive, and the changes are largely dominated by the first F-substitution. On the basis of the calculated proton affinities of CF 2O and CF 3OH, the nucleophilicities of the oxygen atoms are, within computational errors, the same in both compounds.     

Structures and heats of formation of the neutral and ionic PNO, NOP, and NPO systems from electronic structure calculations

High level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set limit have been performed on the PNO, NOP, and NPO isomers and their corresponding anions and cations. Geometries for all species were optimized up through the aug-cc-pV(Q+d)Z level and vibrational frequencies were calculated with the aug-cc-pV(T+d)Z basis set. The most stable of the three isomers is NPO and it is predicted to have a heat of formation of 23.3 kcal/mol. PNO is predicted to be only 1.7 kcal/mol higher in energy. The calculated adiabatic ionization potential of NPO is 12.07 eV and the calculated adiabatic electron affinity is 2.34 eV. The calculated adiabatic ionization potential of PNO is 10.27 eV and the calculated adiabatic electron affinity is only 0.24 eV. NOP is predicted to be much higher in energy by 29.9 kcal/mol. The calculated rotational constants for PNO and NPO should allow for these species to be spectroscopically distinguished. The adiabatic bond dissociation energies for the P[Single Bond]N, P[Single Bond]O, and N[Single Bond]O bonds in NPO and PNO are the same within approximately 10 kcal/mol and fall in the range of 72-83 kcal/mol.     

Third-order Douglas-Kroll relativistic coupled-cluster theory through connected single, double, triple, and quadruple substitutions: applications to diatomic and triatomic hydrides

Coupled-cluster methods including through and up to the connected single, double, triple, and quadruple substitutions have been derived and implemented automatically for sequential and parallel executions by an algebraic and symbolic manipulation program TCE (TENSOR CONTRACTION ENGINE) for use in conjunction with a one-component third-order Douglas-Kroll approximation for relativistic corrections. A combination of the converging electron-correlation methods, the accurate relativistic reference wave functions, and the use of systematic basis sets tailored to the relativistic approximation has been shown to predict the experimental singlet-triplet separations within 0.02 eV (0.5 kcal/mol) for five triatomic hydrides (CH2, NH2+, SiH2, PH2+, and AsH2+), the experimental bond lengths (re or r0) within 0.002 angstroms, rotational constants (Be or B0) within 0.02 cm(-1), vibration-rotation constants (alphae) within 0.01 cm(-1), centrifugal distortion constants (De) within 2%, harmonic vibration frequencies (omegae) within 8 cm(-1) (0.4%), anharmonic vibrational constants (xomegae) within 2 cm(-1), and dissociation energies (D0(0)) within 0.02 eV (0.4 kcal/mol) for twenty diatomic hydrides (BH, CH, NH, OH, FH, AlH, SiH, PH, SH, ClH, GaH, GeH, AsH, SeH, BrH, InH, SnH, SbH, TeH, and IH) containing main-group elements across the second through fifth rows of the periodic table. In these calculations, spin-orbit effects on dissociation energies, which were assumed to be additive, were estimated from the measured spin-orbit coupling constants of atoms and diatomic molecules, and an electronic energy in the complete-basis-set, complete-electron-correlation limit has been extrapolated in two ways to verify the robustness of the results: One assuming Gaussian-exponential dependence of total energies on double through quadruple zeta basis sets and the other assuming n(-3) dependence of correlation energies on double through quintuple zeta basis sets.     

Benchmark calculations on the adiabatic ionization potentials of M-NH(3) (M=Na,Al,Ga,In,Cu,Ag)

The ground states of the M-NH(3) (M=Na,Al,Ga,In,Cu,Ag) complexes and their cations have been studied with density functional theory and coupled cluster [CCSD(T)] methods. The adiabatic ionization potentials (AIPs) of these complexes are calculated, and these are compared to results from high-resolution zero-electron kinetic energy photoelectron spectroscopy. By extrapolating the CCSD(T) energies to the complete basis set (CBS) limit and including the core-valence, scalar relativistic, spin-orbit, and zero-point corrections, the CCSD(T) method is shown to be able to predict the AIPs of these complexes to better than 6 meV or 0.15 kcal/mol. 27 exchange-correlation functionals, including one in the local density approximation, 13 in the generalized gradient approximation (GGA), and 13 with hybrid GGAs, were benchmarked in the calculations of the AIPs. The B1B95, mPW1PW91, B98, B97-1, PBE1PBE, O3LYP, TPSSh, and HCTH93 functionals give an average error of 0.1 eV for all the complexes studied, with the B98 functional alone yielding a maximum error of 0.1 eV. In addition, the calculated metal-ammonia harmonic stretching frequencies with the CCSD(T) method are in excellent agreement with their experimental values, whereas the B3LYP method tends to underestimate these stretching frequencies. The metal-ammonia binding energies were also calculated at the CCSD(T)/CBS level, and are in excellent agreement with the available experimental values considering the error limits, except for Ag-NH(3) and Ag(+)-NH(3), where the calculations predict stronger bond energies than measured by about 4 kcal/mol, just outside the experimental error bars of +/-3 kcal/mol.     

Thermochemistry of the gaseous vanadium chlorides VCl, VCl2, VCl3, and VCl4

Gaseous equilibria in the V-Ag-Cl system were studied at elevated temperatures by effusion-beam mass spectrometry, where the pertinent species were generated by reaction of Cl 2(g) with V + Ag granules in the effusion cell source. Reaction enthalpies were derived from the equilibrium data, and the standard enthalpies of formation at 298 K of gaseous VCl, VCl2, and VCl3 were found to be +49.7, -34.8, and -85.6 kcal mol(-1), respectively. The corresponding bond dissociation energies at 298 K are D(V-Cl) = 102.9 kcal, D(ClV-Cl) = 113.5 kcal, D(Cl2V-Cl) = 79.8 kcal, and D(Cl3V-Cl) = 69.5 kcal. From these data, the dissociation energy D degrees 0(VCl) = 101.9 kcal mol(-1) or 4.42 eV is obtained. An alternate value, Delta(f)H(o)298(VCl 3,g) = -87.0 kcal mol (-1) was derived from third-law analysis of literature sublimation data for VCl3(s). In addition, literature thermochemical data on VCl4(g) were re-evaluated, leading to Delta(f)H(o)298 = -126.1 kcal mol (-1). The results are compared with various estimates in the literature.     

Thermochemical properties of selenium fluorides, oxides, and oxofluorides

The bond dissociation energies (BDEs), fluoride and fluorocation affinities, and electron affinities of SeF(n) (n = 1-6), SeOF(n) (n = 0-4), and SeO(2)F(n) (n = 0-2) have been predicted with coupled cluster CCSD(T) theory extrapolated to the complete basis set limit. To achieve near chemical accuracy, additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies. These included corrections for core-valence effects, scalar relativistic effects, for first-order atomic spin-orbit effects, and vibrational zero point energies. The adiabatic BDEs contain contributions from product reorganization energies and, therefore, can be much smaller than the diabatic BDEs and can vary over a wide range. For thermochemical calculations, the adiabatic values must be used, whereas for bond strength and kinetic considerations, the diabatic values should be used when only small displacements of the atoms without change of the geometry of the molecule are involved. The adiabatic Se-F BDEs of SeF(n) (n = 1-6) are SeF(6) = 90, SeF(5) = 27, SeF(4) = 93, SeF(3) = 61, SeF(2) = 86, and SeF = 76 kcal/mol, and the corresponding diabatic values are SeF(6) = 90, SeF(5) = 88, SeF(4) = 93, SeF(3) = 74, SeF(2) = 86, and SeF = 76 kcal/mol. The adiabatic Se-O BDEs of SeO(n) (n = 1-3), SeOF(n) (n = 1-4), and SeO(2)F(n) (n = 1,2) range from 23 to 107 kcal/mol, whereas the diabatic ones range from 62 to 154 kcal/mol. The adiabatic Se-F BDEs of SeOF(n) (n = 1-4) and SeO(2)F(n) (n = 1,2) range from 20 to 88 kcal/mol, whereas the diabatic ones range from 73 to 112 kcal/mol. The fluoride affinities of SeF(n), (n = 1-6), SeO(n), (n = 1-3), SeOF(n), (n = 1-4), and SeO(2)F(n) (n = 1,2) range from 15 to 121 kcal/mol, demonstrating that the Lewis acidity of these species covers the spectrum from very weak (SeF(6)) to very strong (SeO(3)) acids. The electron affinities which are a measure of the oxidizing power of a species, span a wide range from 1.56 eV in SeF(4) to 5.16 eV in SeF(5) and for the free radicals are much higher than for the neutral molecules. Another interesting feature of these molecules and ions stems from the fact that many of them possess both a Se free valence electron pair and a free unpaired valence electron, raising the questions of their preferred location and their influence on the Se-F and Se═O bond strengths.     

Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).     

Vacuum ultraviolet pulsed field ionization study of ND3: accurate thermochemistry for the ND2-ND2+ and ND3-ND3+ system

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale 相似文献   

氯乙烯的同步幅射光电离及C-H和C-CI键离解能的测定

利用同步辐射对C~2H~2CI进行了光电研究,得到了电离势IP(C~2H~2CI)为10.03±0.01eV及其碎片的光电离出现势,导出了离解能D~0(C~2H~2CI-H),D~0(C~2H~2CI^+-H),D~0(C~2H~2CI-H^+),D~0(C~2H~2-CI)和D~0(C~2H~3^+-CI),它们的值分别为5.09±0.05,5.02±0.08,8.66±0.05,3.64±0.10和2.56±0.05eV.     

Magnitude and nature of interactions in benzene-X (X=ethylene and acetylene) in the gas phase: significantly different CH/pi interaction of acetylene as compared with those of ethylene and methane

The accurate interaction energies of the CH/pi interaction in the benzene-X clusters (X = ethylene and acetylene) were experimentally and theoretically determined. Two-color multiphoton ionization spectroscopy was applied, and the binding energies in the neutral ground state of the clusters were evaluated from the dissociation threshold measurements of the cluster cations. The experimental binding energies of the clusters (D0) were 1.4+/-0.2 and 2.7+/-0.2 kcal/mol, respectively. Estimated CCSD(T) interaction energies for the clusters at the basis set limit (De) were 2.2 and 2.8 kcal/mol, respectively. Calculated D0 values (1.7 and 2.4 kcal/mol, respectively) are close to the experimental values. Large electron correlation contributions (Ecorr=-3.6 and -2.8 kcal/mol, respectively) show that dispersion is the major source of the attraction in both clusters. The electrostatic interaction in the ethylene cluster is very small (-0.38 kcal/mol), as in the case of the benzene-methane cluster, whereas the electrostatic interaction in the acetylene cluster is large (-1.70 kcal/mol). The shifts of the S1-S0 transition also suggest that the ethylene cluster is a van der Waals-type cluster, but the acetylene cluster is a pi-hydrogen-bonded cluster. The nature of the CH/pi interaction of the "activated" alkyne C-H bond is significantly different from that of the "nonactivated" (or typical) alkane and alkene C-H bonds.     

A mass spectrometric and density functional study of the intermetallic molecules AuBe, AuMg, and AuCa

The intermetallic molecules AuBe and AuCa were identified by means of the Knudsen-Effusion Mass Spectrometry technique in the high-temperature vapors produced by vaporizing Au-Be-Ca alloys of proper composition. The gaseous equilibria AuBe(g)+Au(g)=Au(2)(g)+Be(g) and AuCa(g)+Au(g)=Au(2)(g)+Ca(g) were studied in the temperature ranges 1720-1841 K and 1669-1841 K, respectively, by monitoring the partial pressures of all the species involved. The equilibrium data were analyzed by the third-law method, obtaining for the first time the dissociation energy D(0) ( composite function) of the two intermetallic species: D(0) ( composite function)(AuBe)=234.0+/-4.0 kJ/mol; D(0) ( composite function)(AuCa)=246.7+/-4.0 kJ/mol. These values are significantly higher than the recently published D(0) ( composite function) of the species AuMg (175.4+/-2.7 kJ/mol). Furthermore, the ionization energies (IE) of AuBe, AuMg, and AuCa were obtained by measuring the electron impact ionization efficiency curves, IE(AuBe)=7.5+/-0.3 eV, IE(AuMg)=6.7+/-0.3 eV, and IE(AuCa)=5.5+/-0.3 eV. Theoretical calculations were also carried out for these species by density functional theory methods (PW91 and BP86) used in conjunction with Stuttgart relativistic effective core potentials. Both functionals were found to perform very well in reproducing experimental D(0) ( composite function), IE, and molecular parameters.     

Enthalpies of formation of gas-phase N3, N3-, N5+, and N5- from Ab initio molecular orbital theory, stability predictions for N5(+)N3(-) and N5(+)N5(-), and experimental evidence for the instability of N5(+)N3(-)

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.     

A combined zero electronic kinetic energy spectroscopy and ion-pair dissociation imaging study of the F2 + (X 2Pi g) structure

Rotationally resolved pulsed field ionization and zero electronic kinetic energy photoelectron spectra for the transition F(2) (+)(X (2)Pi(g))<--F(2)(X (1)Sigma(g) (+)) have been recorded using the extreme ultraviolet coherence radiation. The vibrational energy spacings, rotational constants, and spin orbit coupling constants for the first three vibrational states of F(2) (+)(X (2)Pi(g)) have been determined accurately. The first adiabatic ionization potential (IP) of F(2) is determined as IP(F(2))=126 585.7+/-0.5 cm(-1). To determine the threshold E(tipp) for ion-pair production of F(2), the images of F(-)((1)S(0)) in the velocity mapping conditions have also been recorded at the photon energy of 126 751 cm(-1). Taking the Stark effect into account, the E(tipp) is determined as E(tipp)(F(2))=126 045+/-8 cm(-1) (15.628+/-0.001 eV). By combing the IP(F(2)) and the E(tipp)(F(2)) determined in this work and together with the reported ionization potential and electronic affinity of the F atom, the bond dissociation energies of F(2) and F(2) (+) are determined as D(0)(F(2))=1.606+/-0.001 eV and D(0)(F(2) (+))=3.334+/-0.001 eV, respectively.     

The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.     

Heats of formation of triplet ethylene, ethylidene, and acetylene

Heats of formation of the lowest triplet state of ethylene and the ground triplet state of ethylidene have been predicted by high level electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T) energies extrapolated to the complete basis set limit using correlation consistent basis sets (CBS), plus additional corrections predict the following heats of formation in kcal/mol: DeltaH0r(C2H4,3A1) = 80.1 at 0 K and 78.5 at 298 K, and DeltaH0t(CH3CH,3A' ') = 86.8 at 0 K and 85.1 at 298 K, with an error of less than +/-1.0 kcal/mol. The vertical and adiabatic singlet-triplet separation energies of ethylene were calculated as DeltaES-T,vert = 104.1 and DeltaES-T,adia = 65.8 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo (DMC) values of 103.5 +/- 0.3 and 66.4 +/- 0.3 kcal/mol. Both sets of computational values differ from the experimental estimate of 58 +/- 3 kcal/mol for the adiabatic splitting. The computed singlet-triplet gap at 0 K for acetylene is DeltaES-T,adia(C2H2) = 90.5 kcal/mol, which is in notable disagreement with the experimental value of 82.6 kcal/mol. The heat of formation of the triplet is DeltaH0tC2H2,3B2) = 145.3 kcal/mol. There is a systematic underestimation of the singlet-triplet gaps in recent photodecomposition experiments by approximately 7 to 8 kcal/mol. For vinylidene, we predict DeltaH0t(H2CC,1A1) = 98.8 kcal/mol at 298 K (exptl. 100.3 +/- 4.0), DeltaH0t(H2CC,3B2) = 146.2 at 298 K, and an energy gap DeltaES-T-adia(H2CC) = 47.7 kcal/mol.     

The dissociation energy of the new diatomic molecules SiPb and GePb

The diatomic molecules SiPb and GePb were for the first time identified by producing high temperature vapors of the constituent pure elements in a "double-oven-like" molecular-effusion assembly. The partial pressures of the atomic, heteronuclear, and homonuclear gaseous species observed in the vapor, namely, Si, Ge, Pb, SiPb, GePb, Pb2, Gen, and Sin (n=2-3), were mass-spectrometrically measured in the overall temperature ranges 1753-1961 K (Ge-Pb) and 1992-2314 K (Si-Pb). The dissociation energies of the new species were determined by second- and third-law analyses of both the direct dissociation reactions and isomolecular exchange reactions involving homonuclear molecules. The selected values of the dissociation energies at 0 K (D0 degrees) are 165.1+/-7.3 and 141.6+/-6.9 kJ/mol, respectively, for SiPb and GePb, and the corresponding enthalpies of formation (DeltafH0 degrees) are 476.4+/-7.3 and 419.3+/-6.9 kJ/mol. The ionization efficiency curves of the two species were measured, giving the following values for the first ionization energies: 7.0+/-0.2 eV (SiPb) and 7.1+/-0.2 eV (GePb). A computational study of the species SiPb and GePb was also carried out at the CCSD(T) level of theory using the relativistic electron core potential approach. Molecular parameters, adiabatic ionization energies, adiabatic electron affinities, and dissociation energies of the title species were calculated, as well as the enthalpy changes of the exchange reactions involving the other Pb-containing diatomics of group 14. Finally, a comparison between the experimental and theoretical results is presented, and from a semiempirical correlation the unknown dissociation energies of the SiSn and PbC molecules are predicted as 234+/-7 and 185+/-11 kJ/mol, respectively.     

Competition between van der Waals and hydrogen bonding interactions: structure of the trans-1-naphthol/N(2) cluster

The excitation energy in the multiphoton ionization spectrum of the trans-1-naphthol/N(2) cluster shows only a small red shift with respect to isolated naphthol, indicating a van der Waals pi-bound structure rather than a hydrogen-bonded one. To confirm this interpretation, high-level electronic structure calculations were performed for several pi- and hydrogen-bonded isomers of this cluster. The calculations were carried out at the second order M?ller-Plesset (MP2) level of perturbation theory with the family of correlation consistent basis sets up to quintuple-zeta quality including corrections for the basis set superposition error and extrapolation to the MP2 complete basis set (CBS) limit. We report the optimal geometries, vibrational frequencies, and binding energies (D(e)), also corrected for harmonic zero-point energies (D(0)), for three energetically low-lying isomers. In all calculations the lowest energy structure was found to be an isomer with the N(2) molecule bound to the pi-system of the naphthol ring carrying the OH group. In the CBS limit its dissociation energy was computed to be D(0) = 2.67 kcal/mol (934 cm(-1)) as compared to D(0) = 1.28 kcal/mol (448 cm(-1)) for the H-bound structure. The electronic structure calculations therefore confirm the assignment of the experimental electronic spectrum corresponding to a van der Waals pi-bound structure. The energetic stabilization of the pi-bound isomer with respect to the hydrogen-bonded one is rather unexpected when compared with previous findings in related systems, in particular phenol/N(2).     

First principles investigation of the electronic structure of the iron carbide cation, FeC+

We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.