Isolated and solvated chiroptical behavior in conformationally flexible butanamines.

The nonresonant optical activity of two highly flexible aliphatic amines, (2R)-3-methyl-2-butanamine (R-MBA) and (2R)-(3,3)-dimethyl-2-butanamine (R-DMBA), has been probed under isolated and solvated conditions to examine the roles of conformational isomerism and to explore the influence of extrinsic perturbations. The optical rotatory dispersion (ORD) measured in six solvents presented uniformly negative rotatory powers over the 320-590 nm region, with the long-wavelength magnitude of chiroptical response growing nearly monotonically as the dielectric constant of the surroundings diminished. The intrinsic specific optical rotation, (in deg dm [g/mL] ), extracted for ambient vapor-phase samples of R-MBA [-11.

Proton-Tunneling Dynamics along Low-Barrier Hydrogen Bonds: A Full-Dimensional Instanton Study of 6-Hydroxy-2-formylfulvene.

Understanding the dynamics of proton transfer along low-barrier hydrogen bonds remains an outstanding challenge of great fundamental and practical interest, reflecting the central role of quantum effects in reactions of chemical and biological importance. Here, we combine calculations with the semiclassical ring-polymer instanton method to investigate tunneling processes on the ground electronic state of 6-hydroxy-2-formylfulvene (HFF), a prototypical neutral molecule supporting low-barrier hydrogen-bonding. The results emerging from a full-dimensional instanton analysis reveal that the tunneling path does not pass through the instantaneous transition-state geometry.

Optical Activity in Saturated Cyclic Amines: Untangling the Roles of Nitrogen-Inversion and Ring-Puckering Dynamics.

The dispersive optical activity of two saturated cyclic amines, ()-2-methylpyrrolidine (-2MPY) and ()-2-methylpiperidine (-2MPI), has been interrogated under isolated and solvated conditions to elucidate the roles of large-amplitude motion associated with nitrogen-center inversion and ring-puckering dynamics. Experimental optical rotatory dispersion profiles were almost mirror images of one another and displayed parallel solvent dependencies. Quantum-chemical analyses built on density-functional and coupled-cluster methods revealed four low-lying conformers for each molecule, which are distinguished by axial/equatorial orientations of their amino hydrogens and methyl substituents.

Hydrogen-Bonding Motifs and Proton-Transfer Dynamics in Electronically Excited 6-Hydroxy-2-formylfulvene.

To elucidate low-barrier hydrogen-bonding (LBHBing) motifs and their ramifications for hydron-migration dynamics, the B-A (π* ← π) absorption system of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFF-) has been probed under free-jet expansion conditions through synergistic application of fluorescence-based laser spectroscopy and quantum-chemical calculations. Neither the donor-acceptor distance nor the proton-transfer barrier is predicted to change markedly between the A and B manifolds, yet a radical alteration in the nature of the reaction coordinate, whereby the planar () transition-state configuration of the former is supplanted by a notably aplanar () form in the latter, is suggested to take place following π* ← π electron promotion (owing, in part, to attendant rearrangements of π-electron conjugation about the molecular framework). In contrast to the strongly perturbed vibrational landscape (commensurate with LBHBing) reported for the A potential surface, the present measurements have revealed surprisingly regular patterns of B vibronic structure which are devoid of obvious band shifts/splittings that would be indicative of efficient proton-transfer processes.

A combined experimental and theoretical study of optical rotatory dispersion for (R)-glycidyl methyl ether in aqueous solution.

The dispersive optical activity for aqueous solutions of non-rigid (R)-glycidyl methyl ether (R-GME) has been explored synergistically from experimental and theoretical perspectives. Density functional theory analyses performed with the polarizable continuum model for implicit solvation identified nine low-lying stable conformers that are interconverted by rotation about two large-amplitude torsional coordinates. The antagonistic chiroptical signatures predicted for these structural isomers were averaged under a Boltzmann-weighting ansatz to estimate the behavior expected for a thermally equilibrated ensemble.

Spectral Signatures of Proton-Transfer Dynamics at the Cusp of Low-Barrier Hydrogen Bonding.

Despite their importance in diverse chemical and biochemical processes, low-barrier hydrogen bonds remain elusive targets to classify and interpret spectroscopically. Here the correlated nature of hydrogen bonding and proton transfer in the low-barrier regime has been probed for the ground and excited electronic states of 6-hydroxy-2-formylfulvene by acquiring jet-cooled fluorescence spectra of the parent and monodeuterated isotopologs. While excited-state profiles reveal regular vibronic patterns devoid of obvious dynamical signatures, their ground-state counterparts display a radically altered energy landscape characterized by spectral bifurcations comparable in magnitude to typical vibrational spacings (>100 cm).

Fixed-Node, Importance-Sampling Diffusion Monte Carlo for Vibrational Structure with Accurate and Compact Trial States.

We present a new approach to importance sampling in diffusion Monte-Carlo (DMC) simulations of vibrational excited states whereby the trial wave functions for low-energy states are incorporated into the diffusion equations so as to enforce their orthogonality. For the model systems examined here, simple variational wave functions based on the vibrational self-consistent field (VSCF) and the simplest vibrational configuration interaction (VCI) are effective in importance sampling provided that internal coordinates used in the underlying one-particle functions have been variationally optimized. The resulting model yields results comparable in accuracy to the best unguided DMC calculations without requiring an a priori choice of coordinates to specify nodal hyperplanes.

Comparison of measured and predicted specific optical rotation in gas and solution phases: A test for the polarizable continuum model of solvation.

A comparative theoretical and experimental study of dispersive optical activity is presented for a set of small, rigid organic molecules in gas and solution phases. Target species were chosen to facilitate wavelength-resolved measurements of specific rotation in rarefied vapors and in organic solvents having different polarities, while avoiding complications due to conformational flexibility. Calculations were performed with two density functionals (B3LYP and CAM-B3LYP) and with the coupled-cluster singles and doubles (CCSD) ansatz, and solvent effects were included through use of the polarizable continuum model (PCM).

Dispersive Optical Activity of (R)-Methylene Norbornene: Intrinsic Response and Solvation Effects.

The dispersive optical activity of a homoconjugated bicyclic diene, (R)-methylene norbornene (R-MNB), was interrogated under complementary vapor-phase and solution-phase conditions to elucidate the structural/electronic provenance of its unusual chiroptical signatures and to explore the marked influence of environmental perturbations. The intrinsic (isolated-molecule) values of specific rotation measured at 355 and 633 nm (1623.5 ± 5.

Dual hydrogen-bonding motifs in complexes formed between tropolone and formic acid.

The near-ultraviolet π←π absorption system of weakly bound complexes formed between tropolone (TrOH) and formic acid (FA) under cryogenic free-jet expansion conditions has been interrogated by exploiting a variety of fluorescence-based laser-spectroscopic probes, with synergistic quantum-chemical calculations built upon diverse model chemistries being enlisted to unravel the structural and dynamical properties of the pertinent ground [X̃A] and excited [ÃAππ] electronic states. For binary TrOH ⋅ FA adducts, the presence of dual hydrogen-bond linkages gives rise to three low-lying isomers designated (in relative energy order) as INT, EXT1, and EXT2 depending on whether docking of the FA ligand to the TrOH substrate takes place internal or external to the five-membered reaction cleft of tropolone. While the symmetric double-minimum topography predicted for the INT potential surface mediates an intermolecular double proton-transfer event, the EXT1 and EXT2 structures are interconverted by an asymmetric single proton-transfer process that is TrOH-centric in nature.

The tropolone-isobutylamine complex: a hydrogen-bonded troponoid without dominant π-π interactions.

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This molecule, which juxtaposes ketonic, hydroxylic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing intermolecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π-π, C-H.

Intrinsic Optical Activity and Large-Amplitude Displacement: Conformational Flexibility in (R)-Glycidyl Methyl Ether.

The dispersive optical activity of (R)-(-)-glycidyl methyl ether (R-GME) has been interrogated under ambient vapor-phase and solution-phase conditions, with quantum-chemical analyses built on density functional (B3LYP and CAM-B3LYP) and coupled-cluster (CCSD) implementations of linear-response theory exploited to interpret experimental findings. Inherent flexibility of the heavy atom skeleton leads to nine low-lying structural isomers that possess distinct chiroptical and physicochemical properties, as evinced by marked changes in the magnitude and the sign of rotatory powers observed in various media. These species are interconverted by independent motion along two large-amplitude torsional coordinates and are stabilized differentially by interaction with the surroundings, thereby reapportioning their relative contributions to the collective response evoked from a thermally equilibrated ensemble.

Insights on the origin of the unusually large specific rotation of (1S,4S)-norbornenone.

Measurements and calculations of specific rotation are indispensable for the characterization of chiral molecules and are now performed routinely. However, the factors that determine the magnitude of this property are still not well-understood. The anomalously large specific rotation of (1S,4S)-norbornenone, an outstanding puzzle for over three decades, offers the chance to examine these factors in detail.

Large solvation effect in the optical rotatory dispersion of norbornenone.

The anomalously large chiroptical response of (1R,4R)-norbornenone has been probed under complementary vapor-phase and solution-phase conditions to assess the putative roles of environmental perturbations. Measurements of the specific rotation for isolated (gas-phase) molecules could not be reproduced quantitatively by comprehensive quantum-chemical calculations based on density-functional or coupled-cluster levels of linear-response theory, which suggests that higher-order treatments may be needed to accurately predict such intrinsic behavior. A substantial, yet unexpected, dependence of the dispersive optical activity on the nature (phase) of the surrounding medium has been uncovered, with the venerable Lorentz local-field correction reproducing solvent-mediated trends in rotatory dispersion surprisingly well, whereas more modern polarizable continuum models for implicit solvation performed less satisfactorily.

Intrinsic optical activity and conformational flexibility: the role of size-dependent ring morphology in model cycloketones.

The optical rotatory dispersion of two monocyclic ketones, (R)-3-methylcyclopentanone [R-3MCP] and (R)-3-methylcyclohexanone [R-3MCH], has been investigated under isolated and solvated conditions to explore the role of ring size/morphology and to elucidate the impact of environmental perturbations. Vapor-phase measurements of specific rotation, [α]λT, were performed at 355/633 nm by means of cavity ring-down polarimetry while complementary solution-phase work employed a canonical discrete-wavelength polarimeter to probe five distinct solvents. The magnitude of [α]λT was found to increase upon solvation, albeit to different extents for the two species of interest, with the attendant sign switching between the solution and vapor phases for λ ≥ 510.

Precursor transformation during molecular oxidation catalysis with organometallic iridium complexes.

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir(III)(chelate)X] series of homogeneous oxidation catalysts. UV-vis and (1)H NMR profiles as well as MALDI-MS data show a rapid and irreversible loss of the Cp* ligand under reaction conditions, which likely proceeds through an intramolecular inner-sphere oxidation pathway reminiscent of the reductive in situ elimination of diolefin placeholder ligands in hydrogenation catalysis by [(diene)M(I)(L,L')](+) (M = Rh and Ir) precursors. When oxidatively stable chelate ligands are bound to the iridium in addition to the Cp*, the oxidized precursors yield homogeneous solutions with a characteristic blue color that remain active in both water- and CH-oxidation catalysis without further induction period.

Towards the accurate and efficient calculation of optical rotatory dispersion using augmented minimal basis sets.

It has been recognized that quantum-chemical predictions of dispersive (nonresonant) chiroptical phenomena are exquisitely sensitive to the periphery of the electronic wavefunction. To further elaborate and potentially exploit this assertion, linear-response calculations of specific optical rotation were performed within the framework of density-functional theory (DFT) by augmenting small basis sets (e.g.

Isotopic dependence of excited-state proton-tunneling dynamics in tropolone probed by polarization-resolved degenerate four-wave mixing spectroscopy.

The origin band of the Ã1B2-X1A1 (π* ← π) absorption system in monodeuterated tropolone (TrOD) has been probed with near-rotational resolution by applying the frequency-domain techniques of polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy under ambient, bulk-gas conditions. Judicious selection of polarization geometries for the incident and detected electromagnetic waves alleviated intrinsic spectral congestion and facilitated dissection of overlapping transitions, thereby enabling refined rotational-tunneling parameters to be extracted for the Ã1B2(π*π) manifold. A tunneling-induced bifurcation of Δ0Ã = 2.

A tale of two carenes: intrinsic optical activity and large-amplitude nuclear displacement.

The specific rotation for two isomeric members of the terpene family, (S)-(+)-2-carene and (S)-(+)-3-carene, has been investigated under complementary solvated and isolated conditions, where the latter vapor-phase work has been performed at excitation wavelengths of 355 and 633 nm by means of ultrasensitive cavity ring-down polarimetry (CRDP). Linear-response computations of dispersive optical activity built upon analogous density-functional (B3LYP/aug-cc-pVTZ) and coupled-cluster (CCSD/aug-cc-pVDZ) levels of theory have been enlisted to unravel the structural and electronic origins of observed behavior. The six-membered portion of the bicyclic skeleton in the nominally rigid 3-carene system is predicted to be near-planar in nature, with calculated and measured rotatory powers for the isolated (gas-phase) species shown to be in excellent agreement.

Vibrational specificity of proton-transfer dynamics in ground-state tropolone.

The vibrational dependence of large-amplitude proton transfer taking place in the ground electronic state (X1A1) of tropolone has been explored by implementing a coherent variant of the stimulated emission pumping (SEP) technique within the framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy. The lowest 1700 cm(-1) portion of this potential surface has been interrogated under ambient bulk-gas conditions, enabling rotationless term energies (Tv+) and tunneling-induced bifurcations Delta(v)X to be extracted for 43 assigned vibrational features of a1 and b2 symmetry. The resulting values of Delta(v)X reflect the state-specificity long attributed to the hydron-migration pathways of tropolone and range in magnitude from 0.

Electronic structure and proton transfer in ground-state hexafluoroacetylacetone.

The ground electronic state (X(1)A(1)) of hexafluoroacetylacetone (HFAA) has been subjected to synergistic experimental and theoretical investigations designed to resolve controversies surrounding the nature of intramolecular hydrogen bonding for the enol tautomer. Cryogenic (93K) X-ray diffraction studies were conducted on single HFAA crystals grown in situ by means of the zone-melting technique, with the resulting electron density maps affording clear evidence for distinguishable O(1)-H and H..

Dissection of rovibronic structure by polarization-resolved two-color resonant four-wave mixing spectroscopy.

A synergistic theoretical and experimental investigation of stimulated emission pumping (SEP) as implemented in the coherent framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy is presented, with special emphasis directed toward the identification of polarization geometries that can distinguish spectral features according to their attendant changes in rotational quantum numbers. A vector-recoupling formalism built upon a perturbative treatment of matter-field interactions and a state-multipole expansion of the density operator allowed the weak-field signal intensity to be cast in terms of a TC-RFWM response tensor, RQ(K)(epsilon4*epsilon3epsilon2*epsilon1;Jg,Je,Jh,Jf), which separates the transverse characteristics of the incident and generated electromagnetic waves (epsilon4*epsilon3epsilon2*epsilon1) from the angular momentum properties of the PUMP and DUMP resonances (Jg,Je,Jh,Jf). For an isolated SEP process induced in an isotropic medium, the criteria needed to discriminate against subsets of rovibronic structure were encoded in the roots of a single tensor element, R0(0)(epsilon4*epsilon3epsilon2*epsilon1;Jg,Je,Jh,Je).

An exploration of electronic structure and nuclear dynamics in tropolone: II. The A (1)B2 (pi* pi) excited state.

The first excited singlet state of tropolone (A (1)B(2)) and the attendant pi(*)<--pi electronic transition have been examined computationally by applying several quantum chemical treatments built upon the aug-cc-pVDZ basis set, including time-dependent density functional theory (TDDFT/B3LYP), configuration interaction singles with perturbative corrections [CIS and CIS(D)], and equation-of-motion coupled-cluster schemes [EOM-CCSD and CR-EOMCCSD(T)]. As in the case of the X (1)A(1) ground state [L. A.