Martin C. R. Cockett

Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen.

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)2(IMes)(py)3]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H2. These reversible processes bring para-H2 and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in 1H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)2(η2-H2)(IMes)(py)2]+, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach.

Vibronic coupling in the ground cationic state of naphthalene: A laser threshold photoelectron [zero kinetic energy (ZEKE)‐photoelectron] spectroscopic study

The two‐color (1+1’) threshold photoelectron spectra of naphthalene in a supersonic free jet have been recorded via nine vibronic levels of the S1 electronic state up to about 1420 cm−1 above the S1 band origin. The threshold photoelectron spectra recorded via the S1 band origin and via totally symmetric ag vibronic levels show significant intensity in Δν=+1 transitions in nontotally symmetric vibrations having b1g symmetry indicating that the ionization transition gains significant intensity through a vibronic coupling mechanism between the two lowest doublet cationic states. The strongest departure from the expected Δν=0 propensity in the threshold photoelectron spectra occurs through excitation of the totally symmetric 8 mode having ag symmetry indicating that a considerable displacement occurs along the normal coordinate of this 8 mode upon ionization from the S1 state. The superior resolution of the threshold photoelectron technique over conventional photoelectron methods has allowed accurate values for the fundamental vibrational frequencies of naphthalene in its ground cationic state to be determined and it has also allowed a more rigorous investigation of the vibronic coupling mechanism that occurs between the two lowest doublet cationic states. Moreover, an improved value for the adiabatic ionization energy of naphthalene of 65 687±7 cm−1 (8.1442±0.0009 eV) has been determined.The two‐color (1+1’) threshold photoelectron spectra of naphthalene in a supersonic free jet have been recorded via nine vibronic levels of the S1 electronic state up to about 1420 cm−1 above the S1 band origin. The threshold photoelectron spectra recorded via the S1 band origin and via totally symmetric ag vibronic levels show significant intensity in Δν=+1 transitions in nontotally symmetric vibrations having b1g symmetry indicating that the ionization transition gains significant intensity through a vibronic coupling mechanism between the two lowest doublet cationic states. The strongest departure from the expected Δν=0 propensity in the threshold photoelectron spectra occurs through excitation of the totally symmetric 8 mode having ag symmetry indicating that a considerable displacement occurs along the normal coordinate of this 8 mode upon ionization from the S1 state. The superior resolution of the threshold photoelectron technique over conventional photoelectron methods has allowed accurate values ...

Evidence for a strong intermolecular bond in the phenol⋅N2 cation

The phenol⋅N2 complex cation has been studied with a combination of two-color resonant zero kinetic energy (ZEKE) and mass analyzed threshold ionization (MATI) spectroscopies to probe the interaction of a polar cation with a quadrupolar solvent molecule. Extended vibrational progressions are observed in three modes which are assigned as the in-plane bend (35 cm−1), the stretch (117 cm−1), and in-plane wag (130 cm−1) intermolecular vibrations, and are consistent with a structure where the N2 forms a directional bond to the phenol OH group in the plane of the aromatic ring. Ab initio calculations at the UMP2/6-31G*, UHF/cc-pVDZ, and UMP2/cc-pVDZ levels of theory support this assignment. The spectra also provide a value for the adiabatic ionization energy (67 423 cm−1±4.5 cm−1) and an estimate of the dissociation energy of the cluster (1650±20 cm−1) which illustrate that the quadrupolar nitrogen molecule binds considerably more strongly to the phenol cation than a rare gas atom. These results constitute the ...

A study of phenylacetylene and styrene, and their argon complexes PA–Ar and ST–Ar with laser threshold photoelectron spectroscopy

In this work, the molecules styrene (ST) and phenylacetylene (PA), as well as their argon complexes ST–Ar and PA–Ar, have been investigated with (1+1’) resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy (TES). The first adiabatic ionization energies of ST, PA, ST–Ar, and PA–Ar have been measured as 68 267±5, 71 175±5, 68 151±5, and 71 027±5 cm−1, respectively. For both ST–Ar and PA–Ar, the first photoelectron band shows structure in the lowest frequency van der Waals (vdW) bending mode in the ground ionic state, with νvdW being measured as 15 cm−1 in each case. For each molecule excitation to a particular vibrational level of the S1 state followed by ionization, allows structure in that mode to be observed in the threshold photoelectron spectrum. This has been achieved for three modes in both styrene and phenylacetylene. The experimental ionic vibrational frequencies thus obtained, have been compared with those known for the S0 and S1 states.

Strategies for the hyperpolarization of acetonitrile and related ligands by SABRE.

We report on a strategy for using SABRE (signal amplification by reversible exchange) for polarizing 1H and 13C nuclei of weakly interacting ligands which possess biologically relevant and nonaromatic motifs. We first demonstrate this via the polarization of acetonitrile, using Ir(IMes)(COD)Cl as the catalyst precursor, and confirm that the route to hyperpolarization transfer is via the J-coupling network. We extend this work to the polarization of propionitrile, benzylnitrile, benzonitrile, and trans-3-hexenedinitrile in order to assess its generality. In the 1H NMR spectrum, the signal for acetonitrile is enhanced 8-fold over its thermal counterpart when [Ir(H)2(IMes)(MeCN)3]+ is the catalyst. Upon addition of pyridine or pyridine-d5, the active catalyst changes to [Ir(H)2(IMes)(py)2(MeCN)]+ and the resulting acetonitrile 1H signal enhancement increases to 20- and 60-fold, respectively. In 13C NMR studies, polarization transfers optimally to the quaternary 13C nucleus of MeCN while the methyl 13C is hardly polarized. Transfer to 13C is shown to occur first via the 1H–1H coupling between the hydrides and the methyl protons and then via either the 2J or 1J couplings to the respective 13Cs, of which the 2J route is more efficient. These experimental results are rationalized through a theoretical treatment which shows excellent agreement with experiment. In the case of MeCN, longitudinal two-spin orders between pairs of 1H nuclei in the three-spin methyl group are created. Two-spin order states, between the 1H and 13C nuclei, are also created, and their existence is confirmed for Me13CN in both the 1H and 13C NMR spectra using the Only Parahydrogen Spectroscopy protocol.

A study of anthracene–Arn (n=0–5) in the ground cationic state by laser threshold photoelectron spectroscopy: Selective ionization of complex isomers formed in the free jet expansion

The van der Waals complexes formed between anthracene and argon in a free jet expansion are studied using laser resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy with the aim of selectively ionizing specific isomers of small‐ to medium‐sized clusters for which discrete absorption peaks exist in the excitation spectrum. Two‐color (1+1’) REMPI threshold photoelectron spectra of a number of isomers of anthracene–Arn (n=1–5) have been recorded in addition to that of anthracene itself. The following adiabatic ionization energies (Ia) have been obtained to within ±5 cm−1: 59 872 (n=0), 59 807 and 59 825 (n=1), 59 757 and 59 774 (n=2), 59 695 (n=3), 59 606 and 59 660 (n=4), and 59 565 cm−1 (n=5). For n=1–3, detailed van der Waals cation vibrational structure was observed, showing progressions in both bending and stretching mode vibrations. The resulting vibrational information together with the ionization energy red shifts has helped in assigning bands observed in the thresho...

Laser threshold photoelectron spectra of the cis and trans rotational isomers of p-dimethoxybenzene-Arn (n=0,1,2) : observation of the intermolecular van der Waals stretching and bending vibrational modes in the cation

The cis and trans rotational isomers of p‐dimethoxybenzene–Arn (n=0,1,2) have been studied in a supersonic free jet by two‐color laser resonance enhanced multiphoton ionization threshold photoelectron spectroscopy. The two‐color (1+1’) threshold photoelectron spectra recorded via the S1 state of the cis and trans isomers of the 1:1 and 1:2 argon complexes reveal well resolved vibrational structure characteristic of the low frequency bending and stretching van der Waals vibrational modes. In the case of the trans isomer of the 1:2 complex, a very low frequency progression (11 cm−1) in a nontotally symmetric van der Waals bending mode appears in single quanta in the spectrum. The equivalent spectrum recorded for the cis isomer exhibits structure characteristic of van der Waals stretching modes as well as double quanta excitation in both totally symmetric and nontotally symmetric van der Waals bending modes. The observation of single quantum excitation in formally forbidden van der Waals vibrational modes im...

The weak hydrogen bond in the fluorobenzene-ammonia van der Waals complex : Insights into the effects of electron withdrawing substituents on π versus in-plane bonding

The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-color resonance enhanced multiphoton ionization (REMPI) spectroscopy, counterpoise corrected RICC2 ab initio molecular orbital calculations, and multidimensional Franck-Condon analysis. The experimental REMPI spectrum is characterized by a dominant, blueshifted band origin, and weak activity in intermolecular vibrational modes. RICC2 geometry optimizations and numerical vibrational frequency calculations of the neutral ground and first excited states have been performed on a number of different structural isomers of the complex using basis sets ranging from augmented double-zeta to quadruple-zeta level. Ground state basis set superposition error corrected zero-point binding energies show the in-plane sigma complex, forming a pseudo-six-membered ring connecting the fluorine atom and ortho-hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable pi and sigma conformers with fluorobenzene show that the sigma complex is the only conformer predicted to exhibit a spectral blueshift upon electronic excitation. The computed neutral ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S(1)-S(0) vibronic spectrum for each of the most stable conformers. These simulations yielded null spectra for transitions involving the most stable of the pi complexes, pi(bridge); a spectrum rich in strong intermolecular vibrational structure for the second of the pi complexes, in complete contrast to the experimental spectrum; and for the sigma complex, a spectrum exhibiting weak intermolecular activity in line with that observed experimentally. This last simulation allowed an almost complete vibrational assignment of the intermolecular structure in the REMPI spectrum. The agreement between computational results and experiment overwhelmingly favors assignment of the spectrum to the in-plane sigma complex.

Observation of torsional motion in the ground-state cation of jet-cooled tolane by two-color threshold photoelectron spectroscopy

High‐resolution threshold photoelectron spectra of jet‐cooled tolane (C6H5–C≡C–C6H5) have been obtained by two‐color (1+1’) resonantly enhanced multiphoton ionization via the S1 state {1B1u(ππ*)}. From the threshold photoelectron spectra, the adiabatic ionization energy has been determined to be Ia = 63 917 ± 3 cm−1 (7.9243±0.0004 eV). A vibrational progression due to torsional motion in the cation ground state was observed up to its fifth quantum level with a fundamental frequency of 54 cm−1, showing small anharmonicity. From these results, we have evaluated the potential barrier height to the torsional motion to be 1980±60 cm−1. The threshold photoelectron spectra obtained via several S1 vibronic levels exhibit dominant peaks due to the Δv=0 ionization transitions. This fact indicates that no significant change occurs in molecular structure upon photoionization with removal of the antibonding π electron from the S1 state. Threshold photoelectron bands arising from formally forbidden odd quantum transiti...

REMPI threshold photoelectron spectra of the cis and trans rotational isomers of jet-cooled m-chlorophenol

Abstract The cis and trans rotational isomers of jet-cooled m -chlorophenol have been studied by two-colour (1+1′) laser threshold photoelectron spectroscopy. The adiabatic ionisation energy ( I a ) for each isomer was obtained from the positions of the respective zeroth vibrational peaks in the threshold photoelectron spectra. This yielded I a values of 8.655±0.001 eV for the cis isomer and 8.682±0.001 eV for the trans isomer. Additionally a provisional assignment of the vibrational structure observed in the ion current spectrum (S 1 state) and the threshold photoelectron spectra (D 0 state) has been made on the basis of comparison with the known ground state vibrational frequencies of the neutral molecule.