Resonant Two-photon Ionization Spectroscopy Research Articles

Positive cooperativity in binding: Spectroscopic characterization and dissociation energy of the anisole…(CH4)2 complex

We have previously reported on the spectroscopy and binding energy of the anisole…methane complex, which exhibits a dual mode of binding that involves both CH/O and CH/π interactions. In this work, we seek to examine cooperativity in binding through a study of the isolated anisole-(methane)2 complex, using a combination of experiments that include mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI) augmented with a complementary theoretical characterization. Using 2CAP and VMI, we derive the dissociation energies of the complex in ground (S0), excited (S1), and cation radical (D0) states. The experimental values from the two methods are in excellent agreement and are compared with selected theoretical values calculated using DFT and ab initio methods. The data show that the dissociation energy increases by some 10 % for the second methane relative to the first, with this trend being consistent across all three electronic states, indicating a cooperative binding effect where the initial solvation turns on binding of a second methane onto the opposite face. This lies in contrast to the aniline-(methane)2 complex, where recent studies have shown a negativity cooperativity, and these trends are examined.

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 0.20eV. A discussion of the periodic trends, summaries of previous work, and comparisons to isoelectronic species is also provided.

A combined spectroscopic and computational investigation on the solvent-to-chromophore excited-state proton transfer in the 2,2'-pyridylbenzimidazole-methanol complex.

This article demonstrates experimental proof of excited state 'solvent-to-chromophore' proton transfer (ESPT) in the isolated gas phase PBI (2,2'-pyridylbenzimidazole)-CH3OH complex, aided by computational calculations. The binary complexes of PBI with CH3OH/CH3OD were produced in a supersonic jet-cooled molecular beam and the energy barrier of the photo-excited process was determined using resonant two-colour two-photon ionization spectroscopy (R2PI). The ESPT process in the PBI-CH3OH complex was confirmed by the disappearance of the Franck-Condon active vibrational transitions above 000 + 390 cm-1. In the PBI-CH3OD complex, the reappearance of the Franck-Condon transitions till 000 + 800 cm-1 confirmed the elevation of the ESPT barrier upon isotopic substitution due to the lowering of the zero-point vibrational energy. The ESPT energy barrier in PBI-CH3OH was bracketed as 410 ± 20 cm-1 (4.91 ± 0.23 kJ mol-1) by comparing the spectra of PBI-CH3OH and PBI-CH3OD. The solvent-to-chromophore proton transfer was confirmed based on the significantly decreased quantum tunnelling of the solvent proton in the PBI-CH3OD complex. The computational investigation resulted in an energy barrier of 6.0 kJ mol-1 for the ESPT reaction in the PBI-CH3OH complex, showing excellent agreement with the experimental value. Overall, the excited state reaction progressed through an intersection of ππ* and nπ* states before being deactivated to the ground state via internal conversion. The present investigation reveals a novel reaction pathway for the deactivation mechanism of the photo-excited N-containing biomolecules in the presence of protic-solvents.

Laser spectroscopic characterization of supersonic jet cooled 2,7-diazaindole.

We report the first gas phase comprehensive study of the electronic spectroscopy of 2,7-diazaindole molecule in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity beyond 2600 cm-1. For the excited state characterization, laser-induced fluorescence (LIF) and two-color resonant two-photon ionization spectroscopy (2C-R2PI) were performed. The band origin (000) for S1 ← S0 transition appeared at 33910 ± 1 cm-1 which was red shifted by 718 cm-1 and 1322 cm-1 compared to that of 7-azaindole and indole respectively. The Franck-Condon active vibrational modes in the spectra were seen till the (000) + 1600 cm-1 region. The IR-UV hole burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. The ionization energy (IE) of the molecule was measured as 8.921 ± 0.001 eV, recorded using photoionization efficiency spectroscopy. The above IE value was significantly higher than that of the related indole derivatives, suggesting the higher photostability of the 27DAI molecule due to N(2) insertion. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectroscopy (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), and the values are 3523 and 3467 cm-1, respectively. The lower value of νNH in the electronic excited state implied the increased photoacidity of the group. A comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theory. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction in comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the lower energy asymmetric modes simulated at the B3LYP/def-SVP level of theory provided the best agreement with the experiment. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. The above study highlights the possibility to appropriately tune the excitation wavelengths as well as alter the photostability of the organic chromophores via additional N-insertion in the molecular systems.

Probing La and Ce Excited-State Reactivity with Resonant Two-Photon Ionization Spectroscopy.

Dehydrogenation and C-C bond cleavage of 1-butyne by the excited states of La and Ce atoms are investigated in laser-ablation metal molecular beams. The excited states of the metal atoms are prepared by resonant excitation, detected by resonant two-photon ionization spectroscopy, and the reaction products are monitored by photoionization time-of-flight mass spectrometry. The reactivities of La* [5d2(3F)6p (4G5/2°)] and Ce* [4f5d(3F°)6s6p(3P°) (5H5)] excited states are observed to be higher than those of the initial states of the corresponding metal atoms. The higher reactivities of the excited states are attributed to their higher energies and favorable electron configurations to form two covalent bonds of the metal-insertion intermediates. Although both excited La and Ce atoms show increased reactivities, the enhancement for Ce is much more pronounced than that of La, which cannot be explained by electron configurations alone. The larger reactivity enhancement from the initial states to the excited state of the Ce atom than that of La is due to the longer lifetime of the Ce excited state.

Ionization energies and cationic bond dissociation energies of RuB, RhB, OsB, IrB, and PtB

Two-photon ionization thresholds of RuB, RhB, OsB, IrB, and PtB have been measured using resonant two-photon ionization spectroscopy in a jet-cooled molecular beam and have been used to derive the adiabatic ionization energies of these molecules. From the measured two-photon ionization thresholds, IE(RuB) = 7.879(9) eV, IE(RhB) = 8.234(10) eV, IE(OsB) = 7.955(9) eV, IE(IrB) = 8.301(15) eV, and IE(PtB) = 8.524(10) eV have been assigned. By employing a thermochemical cycle, cationic bond dissociation energies of these molecules have also been derived, giving D0(Ru+–B) = 4.297(9) eV, D0(Rh+–B) = 4.477(10) eV, D0(Os–B+) = 4.721(9) eV, D0(Ir–B+) = 4.925(18) eV, and D0(Pt–B+) = 5.009(10) eV. The electronic structures of the resulting cationic transition metal monoborides (MB+) have been elucidated using quantum chemical calculations. Periodic trends of the MB+ molecules and comparisons to their neutral counterparts are discussed. The possibility of quadruple chemical bonds in all of these cationic transition metal monoborides is also discussed.

Substitution-induced Nonplanarity of 3-Fluorothioanisole in the First Electronically Excited State.

Vibronic spectra of 3-fluorothioanisole (3FTA) in the first electronic excited state (S1) and the cationic ground state (D0) have been obtained by one-color resonant two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopy. Spectroscopic measurements and theoretical calculations indicate that both cis- and trans-rotamers of the 3FTA molecule are stable and coexist in the S0 (the electronic ground state) and D0 states, and the cis-rotamer is shown to be slightly more stable than the trans-rotamer. In the S1 state, theoretical calculations predict a stable gauche-structure of 3FTA, manifested by the observation of strong activation of the vibrational modes involving the motion of the -SCH3 group in the low-frequency regions of the 1C-R2PI and MATI spectra. The electronic excitation energy from the S0 state to the S1 state (E1) and the adiabatic ionization energy (IE) are respectively determined to be 34 820 ± 3 and 65 468 ± 5 cm-1 for cis-3FTA, and those of the trans-rotamer are respectively determined to be 35 047 ± 3 and 65 644 ± 5 cm-1. The structural properties of the stable rotamers of 3FTA and their comparison with other F- and Cl-substituted thioanisole derivatives are discussed as well.

Molybdenum-Sulfur Bond: Electronic Structure of Low-Lying States of MoS.

The molybdenum-sulfur bond plays an important role in many processes such as nitrogen-fixation, and it is found as a building block in layered materials such as MoS2, known for its various shapes and morphologies. Here, we present an accurate theoretical and experimental investigation of the chemical bonding and the electronic structure of 20 low-lying states of the MoS molecule. Multireference and coupled cluster methodologies, namely, MRCISD, MRCISD + Q, RCCSD(T), and RCCSD[T], were employed in conjunction with basis sets up to aug-cc-pwCV5Z-PP/aug-cc-pwCV5Z for the study of these states. We note the significance of including the inner 4s24p6 electrons of Mo and 2s22p6 of S in the correlated space to obtain accurate results. Experimentally, the predissociation threshold of MoS was measured using resonant two-photon ionization spectroscopy, allowing for a precise measurement of the bond dissociation energy. Our extrapolated computational D0 value for the ground state is 3.936 eV, in excellent agreement with our experimental measurement of 3.932 ± 0.004 eV. The largest calculated adiabatic D0 (5.74 eV) and the largest dipole moment (6.50 D) were found for the 5Σ+ state, where a triple bond is formed. Finally, the connection of the chemical bonding of the isolated MoS species to the relevant solid, MoS2, is emphasized. The low-lying septet states of the diatomic molecule are involved in the material as a building block, explaining the stability and the variety of the shapes and morphologies of the material.

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

We report the spectroscopic observation of the jet-cooled para-ethynylbenzyl (PEB) radical, a resonance-stabilized isomer of C9H7. The radical was produced in a discharge of p-ethynyltoluene diluted in argon and probed by resonant two-color two-photon ionization (R2C2PI) spectroscopy. The origin of the D0(2B1)-D1(2B1) transition of PEB appears at 19,506 cm-1. A resonant two-color ion-yield scan reveals an adiabatic ionization energy (AIE) of 7.177(1) eV, which is almost symmetrically bracketed by CBS-QB3 and B3LYP/6-311G++(d,p) calculations. The electronic spectrum exhibits pervasive Fermi resonances, in that most a1 fundamentals are accompanied by similarly intense overtones or combination bands of non-totally symmetric modes that would carry little intensity in the harmonic approximation. Under the same experimental conditions, the m/z = 115 R2C2PI spectrum of the p-ethynyltoluene discharge also exhibits contributions from the m-ethynylbenzyl and 1-phenylpropargyl radicals. The former, like PEB, is observed herein for the first time, and its identity is confirmed by measurement and calculation of its AIE and D0-D1 origin transition energy; the latter is identified by comparison with its known electronic spectrum (J. Am. Chem. Soc., 2008, 130, 3137-3142). Both species are found to co-exist with PEB at levels vastly greater than might be explained by any precursor sample impurity, implying that interconversion of ethynylbenzyl motifs is feasible in energetic environments such as plasmas and flames, wherein resonance-stabilized radicals are persistent.

Chemical Bonding and Electronic Structure of the Early Transition Metal Borides: ScB, TiB, VB, YB, ZrB, NbB, LaB, HfB, TaB, and WB.

The predissociation thresholds of the early transition metal boride diatomics (MB, M = Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta, W) have been measured using resonant two-photon ionization (R2PI) spectroscopy, allowing for a precise assignment of the bond dissociation energy (BDE). No previous experimental measurements of the BDE exist in the literature for these species. Owing to the high density of electronic states arising from the ground and low-lying separated atom limits in these open d-subshell species, a congested spectrum of vibronic transitions is observed as the energy of the ground separated atom limit is approached. Nonadiabatic and spin-orbit interactions among these states, however, provide a pathway for rapid predissociation as soon as the ground separated atom limit is reached, leading to a sharp decrease in signal to background levels when this limit is reached. Accordingly, the BDEs of the early transition metal borides have been assigned as D0(ScB) 1.72(6) eV, D0(TiB) 1.956(16) eV, D0(VB) 2.150(16) eV, D0(YB) 2.057(3) eV, D0(ZrB) 2.573(5) eV, D0(NbB) 2.989(12) eV, D0(LaB) 2.086(18) eV, D0(HfB) 2.593(3) eV, D0(TaB) 2.700(3) eV, and D0(WB) 2.730(4) eV. Additional insight into the chemical bonding and electronic structures of these species has been achieved by quantum chemical calculations.

Bond dissociation energies of lanthanide sulfides and selenides.

Resonant two-photon ionization spectroscopy has been employed to observe sharp predissociation thresholds in the spectra of the lanthanide sulfides and selenides for the 4f metals Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Lu. As these molecules possess a large density of electronic states near the ground separated atom limit, these predissociation thresholds are argued to coincide with the true 0 K bond dissociation energies (BDEs). This is because spin-orbit and nonadiabatic couplings among these states allow the molecules to predissociate rapidly when the BDE is reached or exceeded. The measured BDEs, in eV, are as follows: 5.230(3) (PrS), 4.820(3) (NdS), 4.011(17) (SmS), 3.811(8) (EuS), 5.282(5) (GdS), 5.292(3) (TbS), 4.298(3) (DyS), 4.251(3) (HoS), 4.262(3) (ErS), 5.189(3) (LuS), 4.496(3) (PrSe), 4.099(3) (NdSe), 3.495(17) (SmSe), 3.319(3) (EuSe), 4.606(3) (GdSe), 4.600(6) (TbSe), 3.602(3) (DySe), 3.562(3) (HoSe), 3.587(3) (ErSe), and 4.599(6) (LuSe). Through the use of thermochemical cycles, the 0 K gaseous heat of formation, ΔfH0K ○, is reported for each molecule. A threshold corresponding to the onset of two-photon ionization in EuSe was also observed, providing the ionization energy of EuSe as 6.483(10) eV. Through a thermochemical cycle and the above reported BDE of the neutral EuSe molecule, the BDE for the Eu+-Se cation was also determined as D0(Eu+-Se) = 2.506(10) eV. Bonding trends of the lanthanide sulfides and selenides are discussed. Our previous observation that the transition metal sulfides are 15.6% more strongly bound than the corresponding selenides continues to hold true for the lanthanides as well.

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.

Probing cooperativity in C-H⋯N and C-H⋯π interactions: Dissociation energies of aniline⋯(CH4)n (n = 1, 2) van der Waals complexes from resonant ionization and velocity mapped ion imaging measurements.

Recent studies of the weakly bound anisole⋯CH4 complex found a dual mode of binding, featuring both C/H⋯π and C/H⋯O noncovalent interactions. In this work, we examine the dissociation energies of related aniline⋯(CH4)n (n = 1, 2) van der Waals clusters, where both C/H⋯π and C/H⋯N interactions are possible. Using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity-mapped ion imaging (VMI), we derive the dissociation energies of both complexes in the ground (S0), excited (S1), and cation radical (D0) states. As the amide group is non-planar in the ground state, the optimized ground state geometry of the aniline⋯CH4 1:1 complex shows two isomers, each with the methane positioned above the aniline ring. The observed redshift of the electronic origin from the aniline monomer is consistent with TDDFT calculations for the more stable isomer, where the methane sits on the same face as the amino hydrogens. The dissociation energies of the 1:1 complex, obtained from 2CAP measurements, are in good agreement with the calculated theoretical values from selected density functional theory methods. VMI data for the 1:1 complex gave a binding energy value overestimated by ∼179 cm-1 when compared to the 2CAP results, indicating that dissociative ionization selectively populates an excited vibrational level of the aniline cation radical. Given that the electron donating ability of aromatic substituents trends as -NH2 > -OCH3 > -CH3, it is noteworthy that the strength of methane binding also trends in this order, as found by experiment (dissociation energies in kJ/mol: 6.6 > 5.8 > 4.5) and predicted by theory (PBE0-D3/def2-QZVPPD, in kJ/mol: 6.9 > 6.0 > 5.0). For the 1:2 complex of aniline and methane, calculations predict that the more stable conformer is the one where the two methane molecules lie on opposite faces of the ring, consistent with the observed redshift of the electronic origin. Unlike the anisole-methane 1:2 complex, which shows an enhanced dissociation energy for the loss of one methane in comparison with the 1:1 complex, here, we find that the energy required to remove one methane from the ground state aniline-methane 1:2 complex is smaller than that of the 1:1 complex, consistent with theoretical expectations.

Bond dissociation energies of the diatomic late transition metal sulfides: RuS, OsS, CoS, RhS, IrS, and PtS.

The spectra of RuS, OsS, CoS, RhS, IrS, and PtS have been recorded near their respective bond dissociation energies using resonant two-photon ionization spectroscopy. The spectra display an abrupt drop to baseline when the bond dissociation energy (BDE) is exceeded. It is argued that spin-orbit and nonadiabatic interactions among the myriad of states that result from the ground and low-lying separated atom limits cause the molecules to predissociate rapidly as soon as the ground separated atom limit is exceeded in energy. Thus, the observed sharp predissociation thresholds are assigned as the 0 K BDEs of the molecules. With this assumption, the BDEs are assigned as follows: 4.071(8) eV (RuS), 4.277(3) eV (OsS), 3.467(5) eV (CoS), 3.611(3) eV (RhS), 4.110(3) eV (IrS), and 4.144(8) eV (PtS). Using thermochemical cycles, the gas-phase enthalpies of formation at 0 K, ΔfH0 K°, were calculated to be 531.8(4.3) kJ mol-1 (RuS), 651.2(6.3) kJ mol-1 (OsS), 365.3(2.2) kJ mol-1 (CoS), 481.5(2.1) kJ mol-1 (RhS), 546.7(6.3) kJ mol-1 (IrS), and 438.9(1.5) kJ mol-1 (PtS). The ionization energies of RuS, CoS, and RhS were also calculated using data on the BDEs of the associated cations and were found to be 8.39(10) eV (RuS), 8.40(9) eV (CoS), and 8.46(12) eV (RhS). Combining these data with predissociation measurements of other transition metal sulfide BDEs, the periodic trends in the transition metal sulfide BDEs are discussed and the BDEs of the transition metal sulfides are compared to those of the corresponding selenides. The BDEs of the sulfides are found to be 15.4% greater than those of the corresponding sulfides.

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 diatomic transition metal selenides: ScSe, YSe, RuSe, OsSe, CoSe, RhSe, IrSe, and PtSe.

The diatomic transition metal selenides, MSe (M = Sc, Y, Ru, Os, Co, Rh, Ir, and Pt), were studied by resonant two-photon ionization spectroscopy near their respective bond dissociation energies. As these molecules exhibit high densities of vibronic states near their dissociation limits, the spectra typically appear quasicontinuously at these energies. Spin-orbit and nonadiabatic couplings among the multitudes of potential curves allow predissociation to occur on a rapid timescale when the molecule is excited to states lying above the ground separated atom limit. This dissociation process occurs so rapidly that the molecules are dissociated before they can be ionized by the absorption of a second photon. This results in an abrupt drop in the ion signal that is assigned as the 0 K bond dissociation energy for the molecule, giving bond dissociation energies of 4.152(3) eV (ScSe), 4.723(3) eV (YSe), 3.482(3) eV (RuSe), 3.613(3) eV (OsSe), 2.971(6) eV (CoSe), 3.039(9) eV (RhSe), 3.591(3) eV (IrSe), and 3.790(31) eV (PtSe). The enthalpies of formation, ΔfH0K° (g), for each diatomic metal selenide were calculated using thermochemical cycles, yielding ΔfH0K° (g) values of 210.9(4.5) kJ mol-1 (ScSe), 203.5(4.5) kJ mol-1 (YSe), 549.2(4.5) kJ mol-1 (RuSe), 675.9(6.5) kJ mol-1 (OsSe), 373.9(2.6) kJ mol-1 (CoSe), 497.4(2.7) kJ mol-1 (RhSe), 557.4(6.5) kJ mol-1 (IrSe), and 433.7(3.6) kJ mol-1 (PtSe). Utilizing a thermochemical cycle, the ionization energy for ScSe is estimated to be about 7.07 eV. The bonding trends of the transition metal selenides are discussed.

Conformer Specific Ultraviolet and Infrared Detection of Nicotine in the Vapor Phase.

A gas-phase electronic spectrum of nicotine in a supersonic expansion has been recorded using two-color resonant two-photon ionization spectroscopy. Efficient photoionization was achievable only via the pyridine chromophore owing to poor Franck-Condon overlap in the N-methylpyrrolidine moiety. Two conformers of nicotine have been characterized and assigned by infrared-ultraviolet (IR-UV) ion depletion and IR-UV hole-burning spectroscopy, in combination with quantum chemical techniques. Trans-A with nitrogen atoms further apart is more stable by 2 kJ mol-1 and the most populated conformer in the supersonic jet, owing this stability to a stronger inter-ring CH···N hydrogen bond than the trans-B counterpart.

Bond dissociation energies of FeB, CoB, NiB, RuB, RhB, OsB, IrB, and PtB.

The bond dissociation energies (BDEs) of the diatomic late transition metal borides (MB, M = Fe, Co, Ni, Ru, Rh, Os, Ir, and Pt) have been assigned from the measurement of a predissociation threshold using resonant two-photon ionization (R2PI) spectroscopy. The open d-shell configurations of the transition metal constituents in the molecules studied here lead to large ML degeneracies, resulting in a dense manifold of states near the ground separated atom limit. This high density of states causes prompt predissociation to occur as soon as the ground separated atom limit is exceeded, allowing a precise assignment of the BDE of the molecule. The measured predissociation thresholds give BDEs of D0(FeB) = 2.43(2) eV, D0(CoB) = 2.954(3) eV, D0(NiB) = 3.431(4) eV, D0(RuB) = 4.815(3) eV, D0(RhB) = 5.252(3) eV, D0(OsB) = 4.378(3) eV, D0(IrB) = 4.928(10) eV, and D0(PtB) = 5.235(3) eV. The gaseous enthalpies of formation at 0 K for these molecules have been derived using a thermochemical cycle that relates atomic enthalpies of formation and the BDE of the molecule, giving ΔfH0K°(g) (FeB) = 733.6(12.2) kJ mol-1, ΔfH0K°(g) (CoB) = 695.1(12.2) kJ mol-1, ΔfH0K°(g) (NiB) = 652.1(14.7) kJ mol-1, ΔfH0K°(g) (RuB) = 740.2(12.7) kJ mol-1, ΔfH0K°(g) (RhB) = 600.1(12.7) kJ mol-1, ΔfH0K°(g) (OsB) = 921.7(13.6) kJ mol-1, ΔfH0K°(g) (IrB) = 748.0(13.6) kJ mol-1, and ΔfH0K°(g) (PtB) = 613.9(12.2) kJ mol-1. This work reports the first experimental measurements of the BDEs of FeB, CoB, NiB, and OsB. Periodic trends are discussed.

Cation spectra of p-chloroanisole and the heavy atom effect on ionization energy

Vibrationally resolved cation spectra of p-chloroanisole were recorded by ionizing via four vibronic levels through two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy techniques. The results indicated that the geometry and molecular vibrations 7a, 16b, and 6a of p-chloroanisole in the cationic ground D0 state resemble those in the electronically excited neutral S1 state. The precisely measured adiabatic ionization energy of this molecule was less than that of p-fluoroanisole by 325 cm−1. This can be labeled as a heavy atom effect, which is similar to that observed for para halogen-substituted phenol and aniline.

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.