Bond dissociation energies of diatomic transition metal nitrides.

Resonant two-photon ionization (R2PI) spectroscopy has been used to measure the bond dissociation energies (BDEs) of the diatomic transition metal nitrides ScN, TiN, YN, MoN, RuN, RhN, HfN, OsN, and IrN. Of these, the BDEs of only TiN and HfN had been previously measured. Due to the many ways electrons can be distributed among the d orbitals, these molecules possess an extremely high density of electronic states near the ground separated atom limit. Spin-orbit and nonadiabatic interactions couple these states quite effectively, so that the molecules readily find a path to dissociation when excited above the ground separated atom limit. The result is a sharp drop in ion signal in the R2PI spectrum when the molecule is excited above this limit, allowing the BDE to be readily measured. Using this method, the values D0(ScN) = 3.905(29) eV, D0(TiN) = 5.000(19) eV, D0(YN) = 4.125(24) eV, D0(MoN) = 5.220(4) eV, D0(RuN) = 4.905(3) eV, D0(RhN) = 3.659(32) eV, D0(HfN) = 5.374(4) eV, D0(OsN) = 5.732(3) eV, and D0(IrN) = 5.115(4) eV are obtained. To support the experimental findings, ab initio coupled-cluster calculations extrapolated to the complete basis set limit (CBS) were performed. With a semiempirical correction for spin-orbit effects, these coupled-cluster single double triple-CBS calculations give a mean absolute deviation from the experimental BDE values of 0.20 eV. A discussion of the periodic trends, summaries of previous work, and comparisons to isoelectronic species is also provided.

The bond dissociation energy of VO measured by resonant three-photon ionization spectroscopy.

The predissociation threshold of VO has been measured using resonant three-photon ionization (R3PI) spectroscopy. Given the high density of electronic states in the molecule, it is argued that the molecule dissociates rapidly as soon as the thermochemical bond dissociation energy (BDE) is exceeded, allowing the measured predissociation threshold to be assigned as the BDE. This is the first time a BDE has been measured using the R3PI method. The first photon is provided by an optical parametric oscillator (OPO) laser that promotes VO into a high-energy, discrete vibronic state. A tunable dye laser then excites the molecule further to a resonant state close to the dissociation limit where there is a quasi-continuum of states. A second photon from the same dye laser pulse ionizes the molecule, generating VO+ ions. The dye laser is then scanned to higher energies, and when the energy of one OPO photon plus one dye photon exceeds the BDE, the molecule dissociates before another dye photon can be absorbed to induce ionization. The combined photon energy at the sharp drop in the ion signal is assigned as the BDE. The experiment has been repeated using four different intermediate states, all yielding the same BDE, D0(VO) = 6.545(2) eV. Using thermochemical cycles, a revised value for the BDE of cationic VO is obtained, D0(V+-O) = 6.053(2) eV. The 0 K enthalpy of formation for VO(g) is also derived as ΔfH0K 0VO(g) = 128.6(1.0) kJ mol-1. Previous spectroscopic and thermochemical studies of VO are reviewed.

Bond dissociation energies of diatomic transition metal sulfides: ScS, YS, TiS, ZrS, HfS, NbS, and TaS.

The early transition metal diatomic sulfides, MS, M = Sc, Y, Ti, Zr, Hf, Nb, and Ta, have been investigated using resonant two-photon ionization spectroscopy in the vicinity of their bond dissociation energies (BDEs). Due to the high density of vibronic states in this energy range, the molecular spectra appear quasicontinuous, and when the excitation energy exceeds the ground separated atom limit, excited state decay by dissociation becomes possible. The dissociation process typically occurs so rapidly that the molecule falls apart before a second photon can be absorbed to ionize the species, leading to a sharp drop in ion signal, which is identified as the 0 K BDE. The observed predissociation thresholds yield BDEs of 4.852(10) eV (ScS), 5.391(3) eV (YS), 4.690(4) eV (TiS), 5.660(4) eV (ZrS), 5.780(20) eV (HfS), 5.572(3) eV (NbS), and 5.542(3) eV (TaS). Utilizing thermochemical cycles, the enthalpies of formation, ΔfH0K o(g), of 182.7(4.3) kJ mol-1 (ScS), 178.3(4.2) kJ mol-1 (YS), 293.1(16.7) kJ mol-1 (TiS), 337.3(8.4) kJ mol-1 (ZrS), 335.0(6.6) kJ mol-1 (HfS), 467.0(8.0) kJ mol-1 (NbS), and 521.5(2.1) kJ mol-1 (TaS) are obtained. Another thermochemical cycle has been used to combine the previously measured M+-S BDEs with the M-S BDEs and atomic ionization energies to obtain the MS ionization energies of 6.44(5) eV (ScS), 6.12(8) eV (YS), 6.78(7) eV (TiS), 6.60(10) eV (ZrS), and 6.88(9) eV (NbS). Using this same cycle, we obtain D0(Hf+-S) = 4.926(20) eV. The bonding trends of the early transition metal sulfides, along with the corresponding selenides, are discussed.

Bond dissociation energies of transition metal oxides: CrO, MoO, RuO, and RhO.

Through the use of resonant two-photon ionization spectroscopy, sharp predissociation thresholds have been identified in the spectra of CrO, MoO, RuO, and RhO. Similar thresholds have previously been used to measure the bond dissociation energies (BDEs) of many molecules that have a high density of vibronic states at the ground separated atom limit. A high density of states allows precise measurement of the BDE by facilitating prompt dissociation to ground state atoms when the BDE is exceeded. However, the number of states required for prompt predissociation at the thermochemical threshold is not well defined and undoubtedly varies from molecule to molecule. The ground separated atom limit generates 315 states for RuO, 252 states for RhO, and 63 states for CrO and MoO. Although comparatively few states derive from this limit for CrO and MoO, the observation of sharp predissociation thresholds for all four molecules nevertheless allows BDEs to be assigned as 4.863(3) eV (RuO), 4.121(3) eV (RhO), 4.649(5) eV (CrO), and 5.414(19) eV (MoO). Thermochemical cycles are used to derive the enthalpies of formation of the gaseous metal oxides and to obtain IE(RuO) = 8.41(5) eV, IE(RhO) = 8.56(6) eV, D0(Ru-O-) = 4.24(2) eV, D0(Cr-O-) = 4.409(8) eV, and D0(Mo-O-) = 5.243(20) eV. The mechanisms leading to prompt predissociation at threshold in the cases of CrO and MoO are discussed. Also presented is a discussion of the bonding trends for the transition metal oxides, which are compared to the previously measured transition metal sulfides.

Bond dissociation energies of ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH.

Predissociation thresholds of the ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH molecules have been measured using resonant two-photon ionization spectroscopy. It is argued that the dense manifold of electronic states present in these molecules causes prompt dissociation when the bond dissociation energy (BDE) is exceeded, allowing their respective predissociation thresholds to provide precise values of their bond energies. The BDEs were measured as 2.015(3) eV (ScSi), 2.450(2) eV (YSi), 2.891(5) eV (LaSi), 3.042(10) eV (ScC), 3.420(3) eV (YC), 4.718(4) eV (LaC), 3.899(13) eV (CoC), and 4.102(3) eV (Y-CH). Using thermochemical cycles, the enthalpies of formation, ΔfH0K°(g), were calculated as 627.4(9.0) kJ mol-1 (ScSi), 633.1(9.0) kJ mol-1 (YSi), 598.1(9.0) kJ mol-1 (LaSi), 793.8(4.3) kJ mol-1 (ScC), 805.0(4.2) kJ mol-1 (YC), 687.3(4.2) kJ mol-1 (LaC), 760.1(2.5) kJ mol-1 (CoC), and 620.8(4.2) kJ mol-1 (YCH). Using data for the BDEs of the corresponding cations allows ionization energies to be obtained through thermochemical cycles as 6.07(11) eV (ScSi), 6.15(13) eV (YSi), 5.60(10) eV (LaSi), 6.26(6) eV (ScC), 6.73(12) or 5.72(11) eV [YC, depending on the value of D0(Y+-C) employed], and 5.88(35) eV (LaC). Additionally, a new value of D0(Co+-C) = 4.045(13) eV was obtained based on the present work and the previously determined ionization energy of CoC. An ionization onset threshold allowed the measurement of the LaSi ionization energy as 5.607(10) eV, in excellent agreement with a prediction based on a thermochemical cycle. Chemical bonding trends are also discussed.

Bond dissociation energies of FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi.

Resonant two-photon ionization spectroscopy has been used to investigate the spectra of the diatomic late transition metal silicides, MSi, M = Fe, Ru, Os, Co, Rh, Ir, Ni, and Pt, in the vicinity of the bond dissociation energy. In these molecules, the density of vibronic states is so large that the spectra appear quasicontinuous in this energy range. When the excitation energy exceeds the ground separated atom limit, however, a new decay process becomes available-molecular dissociation. This occurs so rapidly that the molecule falls apart before it can absorb another photon and be ionized. The result is a sharp drop to the baseline in the ion signal, which we identify as occurring at the thermochemical 0 K bond dissociation energy, D0. On this basis, the measured predissociation thresholds provide D0 = 2.402(3), 4.132(3), 4.516(3), 2.862(3), 4.169(3), 4.952(3), 3.324(3), and 5.325(9) eV for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Using thermochemical cycles, the enthalpies of formation of the gaseous MSi molecules are derived as 627(8), 700(10), 799(10), 595(8), 599(8), 636(10), 553(12), and 497(8) kJ/mol for FeSi, RuSi, OsSi, CoSi, RhSi, IrSi, NiSi, and PtSi, respectively. Likewise, combining these results with other data provides the ionization energies of CoSi and NiSi as 7.49(7) and 7.62(7) eV, respectively. Chemical bonding trends among the diatomic transition metal silicides are discussed.

Bond dissociation energies of TiC, ZrC, HfC, ThC, NbC, and TaC.

Sharp predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiC, ZrC, HfC, ThC, NbC, and TaC. Because of the large density of states in these species, particularly near the ground separated atom limit, we argue that the sharp predissociation threshold occurs at the thermochemical bond dissociation energy. The bond dissociation energies, D0(MC), measured are 3.857(4) eV (TiC), 4.892(10) eV (ZrC), 4.426(3) eV (HfC), 5.060(3) eV (ThC), 5.620(4) eV (NbC), and 4.975(3) eV (TaC). Using atomic enthalpies of formation, the diatomic enthalpies of formation, Δf,0KH○(MC(g)), were also calculated as 810.0(16.7) kJ mol-1 (TiC), 847.9(8.5) kJ mol-1 (ZrC), 902.1(6.3) kJ mol-1 (HfC), 825.0(6.0) kJ mol-1 (ThC), 898.8(8.0) kJ mol-1 (NbC), and 1012.6(2.2) kJ mol-1 (TaC). Combining our D0(MC) values with accurate values of the ionization energies of MC and M, we also report precise values of D0(Ti+-C) = 4.089(4) eV, D0(V+-C) = 3.724(3) eV, and D0(Nb+-C) = 5.390(4) eV. Combining the present D0(MC) results with guided ion beam measurements of cationic bond dissociation energies, we report MC ionization energies of IE(ZrC) = 6.91(16) eV, IE(HfC) = 8.06(3) eV, IE(ThC) = 6.55(29) eV, and IE(TaC) = 8.73(4) eV. Trends in the transition metal MC bond energies and a comparison to MSi bond energies are also presented.

Bond Dissociation Energies of Tungsten Molecules: WC, WSi, WS, WSe, and WCl.

Resonant two-photon ionization spectroscopy was used to locate predissociation thresholds in WC, WSi, WS, WSe, and WCl, allowing bond dissociation energies to be measured for these species. Because of the high degree of vibronic congestion in the observed spectra, it is thought that the molecules dissociate as soon as the lowest separated atom limit is exceeded. From the observed predissociation thresholds, dissociation energies are assigned as D0(WC) = 5.289(8) eV, D0(WSi) = 3.103(10) eV, D0(WS) = 4.935(3) eV, D0(WSe) = 4.333(6) eV, and D0(WCl) = 3.818(6) eV. These results are combined with other data to obtain the ionization energy IE(WC) = 8.39(9) eV and the anionic bond dissociation energies of D0(W-C-) = 6.181(17) eV, D0(W--C) = 7.363(19) eV, D0(W-Si-) ≤ 3.44(4) eV, and D0(W--Si) ≤ 4.01(4) eV. Combination of the D0(WX) values with atomic enthalpies of formation also provides ΔfH0K° values for the gaseous WX molecules. Computational results are also provided, which shed some light on the electronic structure of these molecules.

Bond dissociation energies of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D0(TiSi) = 2.201(3) eV, D0(ZrSi) = 2.950(3) eV, D0(HfSi) = 2.871(3) eV, D0(VSi) = 2.234(3) eV, D0(NbSi) = 3.080(3) eV, and D0(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as Δf,0KH°(TiSi(g)) = 705(19) kJ mol-1, Δf,0KH°(ZrSi(g)) = 770(12) kJ mol-1, Δf,0KH°(HfSi(g)) = 787(10) kJ mol-1, Δf,0KH°(VSi(g)) = 743(11) kJ mol-1, Δf,0KH°(NbSi(g)) = 879(11) kJ mol-1, and Δf,0KH°(TaSi(g)) = 938(8) kJ mol-1. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi- and NbSi- anions, D0(Zr-Si-) ≤ 3.149(15) eV, D0(Zr--Si) ≤ 4.108(20) eV, D0(Nb-Si-) ≤ 3.525(31) eV, and D0(Nb--Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.

The bond length and bond energy of gaseous CrW.

Supersonically cooled CrW was studied using resonant two-photon ionization spectroscopy. The vibronically resolved spectrum was recorded over the region 21 100 to 23 400 cm(-1), showing a very large number of bands. Seventeen of these bands, across three different isotopologues, were rotationally resolved and analyzed. All were found to arise from the ground (1)Σ(+) state of the molecule and to terminate on states with Ω' = 0. The average r0 bond length across the three isotopic forms was determined to be 1.8814(4) Å. A predissociation threshold was observed in this dense manifold of vibronic states at 23 127(10) cm(-1), indicating a bond dissociation energy of D0(CrW) = 2.867(1) eV. Using the multiple bonding radius determined for atomic Cr in previous work, the multiple bonding radius for tungsten was calculated to be 1.037 Å. Comparisons are made between CrW and the previously investigated group 6 diatomic metals, Cr2, CrMo, and Mo2, and to previous computational studies of this molecule. It is also found that the accurately known bond dissociation energies of group 5/6 metal diatomics Cr2, V2, CrW, NbCr, VNb, Mo2, and Nb2 display a qualitative linear dependence on the sum of the d-orbital radial expectation values, r; this relationship allows the bond dissociation energies of other molecules of this type to be estimated.

Bond dissociation energies of diatomic transition metal selenides: TiSe, ZrSe, HfSe, VSe, NbSe, and TaSe.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSe, ZrSe, HfSe, VSe, NbSe, and TaSe. It is argued that the sharp onset of predissociation corresponds to the bond dissociation energy in each of these molecules due to their high density of states as the ground separated atom limit is approached. The bond dissociation energies obtained are D0(TiSe) = 3.998(6) eV, D0(ZrSe) = 4.902(3) eV, D0(HfSe) = 5.154(4) eV, D0(VSe) = 3.884(3) eV, D0(NbSe) = 4.834(3) eV, and D0(TaSe) = 4.705(3) eV. Using these dissociation energies, the enthalpies of formation were found to be Δf,0KHo(TiSe(g)) = 320.6 ± 16.8 kJ mol-1, Δf,0KHo(ZrSe(g)) = 371.1 ± 8.5 kJ mol-1, Δf,0KHo(HfSe(g)) = 356.1 ± 6.5 kJ mol-1, Δf,0KHo(VSe(g)) = 372.9 ± 8.1 kJ mol-1, Δf,0KHo(NbSe(g)) = 498.9 ± 8.1 kJ mol-1, and Δf,0KHo(TaSe(g)) = 562.9 ± 1.5 kJ mol-1. Comparisons are made to previous work, when available. Also reported are calculated ground state electronic configurations and terms, dipole moments, vibrational frequencies, bond lengths, and bond dissociation energies for each molecule. A strong correlation of the measured bond dissociation energy with the radial expectation value, ⟨r⟩nd, for the metal atom is found.