Scott A. Reid

The halocarbenes: model systems for understanding the spectroscopy, dynamics and chemistry of carbenes

We review recent studies of the spectroscopy and dynamics of halocarbenes :CXY (X = H, F, Cl, Br, I; Y = F, Cl, Br, I), which are set forth as model systems for understanding the spectroscopy, photochemistry and photophysics of carbenes, and benchmarks for comparing experiment and theory concerning carbene singlet–triplet gaps and spin–orbit coupling from the limit of mixing of isolated rovibronic levels to energy perturbations involving entire vibronic levels. Following a historical overview of halocarbene chemistry, which spans more than 50 years, we discuss experimental methods for generating and spectroscopically detecting these elusive species. Subsequent sections focus on: (a) the spectroscopy of the halocarbenes, where a variety of non-adiabatic interactions are operative, (b) the singlet–triplet gap in the halocarbenes, where experimental methods for measuring this quantity are outlined and their results are compared with benchmark theoretical calculations and (c) the photochemistry and photodissociation dynamics of the halocarbenes, which emphasises recent observations of the hitherto unobserved quasilinear B state.

Fluorescence excitation and emission spectroscopy of the ÃA″1←X̃A′1 system of CHBr

We report fluorescence excitation and emission spectra of CHBr in the 450–750nm region. A total of 30 cold bands involving the pure bending levels 20n with n=2–8 and combination bands 20n301(n=1–8), 20n302(n=1–6), 20n303(n=1–2), 10120n(n=5–7), 10120n301(n=4–6), and 10120n302(n=5) in the AA″1←XA′1 system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers (CHBr79,CHBr81). The derived AA″1 vibrational intervals are combined with results of [Yu et al. J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The AA″1 state C–H stretching frequency is determined here for the first time, and the observed ν3 dependence of the Br79–Br81 isotope splitting in the AA″1 state is in good agreement ...

π-Stacking, C–H/π, and Halogen Bonding Interactions in Bromobenzene and Mixed Bromobenzene–Benzene Clusters

Noncovalent interactions play an important role in many chemical and biochemical processes. Building upon our recent study of the homoclusters of chlorobenzene, where π-π stacking and CH/π interactions were identified as the most important binding motifs, in this work we present a study of bromobenzene (PhBr) and mixed bromobenzene-benzene clusters. Electronic spectra in the region of the PhBr monomer S0-S1 (ππ*) transition were obtained using resonant two-photon ionization (R2PI) methods combined with time-of-flight mass analysis. As previously found for related systems, the PhBr cluster spectra show a broad feature whose center is red-shifted from the monomer absorption, and electronic structure calculations indicate the presence of multiple isomers and Franck-Condon activity in low-frequency intermolecular modes. Calculations at the M06-2X/aug-cc-pVDZ level find in total eight minimum energy structures for the PhBr dimer: four π-stacked structures differing in the relative orientation of the Br atoms (denoted D1-D4), one T-shaped structure (D5), and three halogen bonded structures (D6-D8). The calculated binding energies of these complexes, corrected for basis set superposition error (BSSE) and zero-point energy (ZPE), are in the range of -6 to -24 kJ/mol. Time-dependent density functional theory (TDDFT) calculations predict that these isomers absorb over a range that is roughly consistent with the breadth of the experimental spectrum. To examine the influence of dipole-dipole interaction, R2PI spectra were also obtained for the mixed PhBr···benzene dimer, where the spectral congestion is reduced and clear vibrational structure is observed. This structure is well-simulated by Franck-Condon calculations that incorporate the lowest frequency intermolecular modes. Calculations find four minimum energy structures for the mixed dimer and predict that the binding energy of the global minimum is reduced by ~30% relative to the global minimum PhBr dimer structure.

Characterization of iso-CF2I2 in frequency and ultrafast time domains

The photolysis of diiododifluoromethane (CF(2)I(2)) in condensed phases was studied by a combination of matrix isolation and ultrafast time-resolved spectroscopy, in concert with ab initio calculations. Photolysis at wavelengths of 355 or 266 nm of CF(2)I(2):Ar samples (1:5000) held at approximately 8 K yielded iso-CF(2)I(2) (F(2)C-I-I), a metastable isomer of CF(2)I(2), characterized here for the first time. The infrared (IR) spectra of this isomer were recorded in matrix experiments, and the derived positions of the C-F stretching modes are in very good agreement with the predictions of high level ab initio calculations, which show that the iso-form is a minimum on the CF(2)I(2) ground state potential energy surface. The formation of this isomer following 350 nm excitation of CF(2)I(2) in room temperature CCl(4) solutions was monitored through its intense C-F stretching mode by means of ultrafast time-resolved IR absorption. Together, matrix isolation and ultrafast IR absorption experiments suggest that the formation of iso-CF(2)I(2) occurs via recombination of CF(2)I radical and I atom. Ultrafast IR experiments detect a delayed rise of iso-CF(2)I-I absorption, placing an upper limit of 400 fs for the C-I bond dissociation and primary geminate recombination processes. The product absorption spectrum recorded 1 ns after 350 nm excitation of CF(2)I(2) in solution is virtually identical to the visible absorption spectrum of iso-CF(2)I(2) trapped in matrix isolation experiments [with subtracted I(2)(X) absorption]. The formation of this isomer in solution at room temperature has direct dynamic implications for the ultrafast production of molecular iodine from electronically excited CF(2)I(2).

Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm

The correlated speed and rotational energy distributions of the CO fragment from photodissociation of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distributions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ > 325 nm. This threshold corresponds to C–H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the presence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.

Spectroscopic and computational studies of matrix-isolated iso-CHBr3: Structure, properties, and photochemistry of iso-bromoform

Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.

On π-stacking, C-H/π, and halogen bonding interactions in halobenzene clusters: resonant two-photon ionization studies of chlorobenzene.

Noncovalent interactions such as hydrogen bonding, π-π stacking, CH/π interactions, and halogen bonding play crucial roles in a broad spectrum of chemical and biochemical processes, and can exist in cooperation or competition. Here we report studies of the homoclusters of chlorobenzene, a prototypical system where π-π stacking, CH/π interactions, and halogen bonding interactions may all be present. The electronic spectra of chlorobenzene monomer and clusters (Clbz)(n) with n = 1-4 were obtained using resonant 2-photon ionization in the origin region of the S(0)-S(1) (ππ*) state of the monomer. The cluster spectra show in all cases a broad spectrum whose center is redshifted from the monomer absorption. Electronic structure calculations aid in showing that the spectral broadening arises in large part from inhomogeneous sources, including the presence of multiple isomers and Franck-Condon (FC) activity associated with geometrical changes induced by electronic excitation. Calculations at the M06-2x/aug-cc-pVDZ level find in total five minimum energy structures for the dimer, four π-stacked structures, and one T-shaped, and six representative minimum energy structures were found for the trimer. The calculated time-dependent density functional theory spectra using range-separated and meta-GGA hybrid functionals show that these isomers absorb over a range that is roughly consistent with the breadth of the experimental spectra, and the calculated absorptions are redshifted with respect to the monomer transition, in agreement with experiment. Due to the significant geometry change in the electronic transition, where for the dimer a transition from a parallel displaced to sandwich structure occurs with a reduced separation of the two monomers, significant FC activity is predicted in low frequency intermolecular modes.

Infrared degenerate four wave mixing spectroscopy of jet-cooled C2H2

Abstract We report the application of infrared degenerate four-wave mixing (IR DFWM) spectroscopy to probe rotational-vibrational transitions in the v 3 / v 2 + v 4 + v 5 Fermi dyad region of jet-cooled C 2 H 2 . The spectral intensities show a quadratic population dependence, and are well reproduced by a Boltzmann distribution of J state populations at T rot = 25 K. Intensity dependence measurements and lineshape analysis show a cubic dependence of the DFWM signal on laser intensity, as expected from theory in the weak field limit ( I ⪡ I sat ). We compare DFWM signals in jet versus bulk experiments.

Photoisomerization and photoinduced reactions in liquid CCl4 and CHCl3.

Transient absorption spectroscopy is used to follow the reactive intermediates involved in the first steps in the photochemistry initiated by ultraviolet (266-nm wavelength) excitation of solutions of 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in carbon tetrachloride or chloroform. Ultraviolet and visible bands centered close to 330 and 500 nm in both solvents are assigned respectively to a charge transfer band of Cl-solvent complexes and the strong absorption band of a higher energy isomeric form of the solvent molecules (iso-CCl3-Cl or iso-CHCl2-Cl). These assignments are supported by calculations of electronic excitation energies. The isomeric forms have significant contributions to their structures from charge-separated resonance forms and offer a reinterpretation of previous assignments of the carriers of the visible bands that were based on pulsed radiolysis experiments. Kinetic analysis demonstrates that the isomeric forms are produced via the Cl-solvent complexes. Addition of the unsaturated hydrocarbons provides a reactive loss channel for the Cl-solvent complexes, and reaction radii and bimolecular rate coefficients are derived from analysis using a Smoluchowski theory model. For reactions of Cl with 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in CCl4, rate coefficients at 294 K are, respectively, (8.6 ± 0.8) × 10(9), (9.5 ± 1.6) × 10(9), and (1.7 ± 0.1) × 10(10) M(-1) s(-1). The larger reaction radius and rate coefficient for 2,3-dimethylbut-2-ene are interpreted as evidence for an H-atom abstraction channel that competes effectively with the channel involving addition of a Cl-atom to a C═C bond. However, the addition mechanism appears to dominate the reactions of 1,5-hexadiene and isoprene. Two-photon excited CCl4 or CHCl3 can also ionize the diene or alkene solute.

Formation and relaxation dynamics of iso-CH2Cl–I in cryogenic matrices

Photolysis of chloroiodomethane (CH(2)ClI) in cryogenic matrices followed by recombination of the nascent radical pair produces an isomer (CH(2)Cl-I) that features a halogen-halogen (Cl-I) bond. Using ultrafast laser pulses, it is possible to follow the formation of this isomer by transient electronic absorption in low-temperature matrices of N(2), CH(4), and Ar. Frequency-domain measurements provide vibrational and electronic spectra, and electronic structure calculations give the structures of the isomers and the minimum energy path that connects them. The ultrafast experiments cleave the C-I bond with a 267-nm photolysis pulse and probe the formation of the isomer at wavelengths between 435 nm and 510 nm. The longest wavelengths preferentially interrogate vibrationally excited molecules, and their transient absorption shows that the highly vibrationally excited isomer appears within 1 to 2 ps, depending on the matrix, likely reflecting the loss of 2000 cm(-1) or more of energy in a strong, inelastic collision of the fragments with the matrix. The subsequent relaxation of the vibrationally excited isomer occurs in 20 to 40 ps, a time that is comparable to those observed for halomethane molecules and their isomers in liquids and in supercritical CO(2). These observations suggest that the formation and initial relaxation of the isomer in dense media do not depend strongly on the identity of the surroundings.