Vibronic spectroscopy of a nitrile/isonitrile isoelectronic pair: para-diisocyanobenzene and para-isocyanobenzonitrile.

The ultraviolet spectroscopy of isoelectronic pair para-diisocyanobenzene (pDIB) and para-isocyanobenzonitrile (pIBN) has been studied under gas-phase, jet-cooled conditions. These molecules complete a sequence of mono and disubstituted nitrile/isonitrile benzene derivatives, enabling a comparison of the electronic effects of such substitution. Utilizing laser-induced fluorescence (LIF) and resonant two-photon ionization (R2PI) spectroscopy, the S0-S1 electronic origins of pDIB and pIBN have been identified at 35,566 and 35,443 cm(-1), respectively. In pDIB, the S0-S1 origin is very weak, with b(3g) fundamentals induced by vibronic coupling to the S2 state dominating the spectrum at 501 cm(-1) (ν17, isocyano bend) and 650 cm(-1) (ν16, ring distortion). The spectrum extends over 5000 cm(-1), remaining sharp and relatively uncongested over much of this range. Dispersed fluorescence (DFL) spectra confirm the dominating role played by vibronic coupling and identify Franck-Condon active ring modes built off the vibronically-induced bands. In pDIB, the S2 state has been tentatively observed at about 6100 cm(-1) above the S0-S1 origin. In pIBN, the S0-S1 origin is considerably stronger, but vibronic coupling still plays an important role, involving fundamentals of b2 symmetry. The bending mode of the nitrile group dominates the vibronically-induced activity. Calculations carried out at the TD-DFT B3LYP/6-31+G(d) level of theory account for the extremely weak S0-S1 oscillator strength of pDIB and the larger intensity of the S0-S1 origins of pIBN and pDCB (para-dicyanobenzene) as nitrile groups are substituted for isonitrile groups. In pDIB, a nearly perfect cancellation of transition dipoles occurs due to two one-electron transitions that contribute nearly equally to the S0-S1 transition. The spectra of both molecules show no clear evidence of charge-transfer interactions that play such an important role in some cyanobenzene derivatives.

Conformation-specific spectroscopy and populations of diastereomers of a model monolignol derivative: chiral effects in a triol chain.

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(ß), thereby incorporating key aspects of the ß-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-ß-γ, γ-ß-α, and α-γ-ß-π chains and two cyclic forms [(αßγ) and (αγß)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-ß-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-ß-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.

High-resolution IR spectroscopy of dimers of HDO with H2O in helium nanodroplets.

We report on rotationally resolved IR spectra of dimers of HDO as a deuterium (d) donor with H(2)O, HDO, and D(2)O embedded in superfluid Helium nanodroplets in the 2650-2660 and 2725-2740 cm(-1) regions of the O-D donor stretch and symmetric acceptor stretch vibrations, respectively. By comparing spectra at different levels of deuteration we were able to unambiguously assign the donor stretch signals of H(2)O···DOH, HDO···DOH, and D(2)O···DOH. For H(2)O···DOH, three ΔK(a) = 0 sub-bands were found that were assigned to transitions from the lower and upper acceptor switching states of K(a) = 0 and the lower acceptor switching state of K(a) = 1. In addition, b- and c-type transitions in the acceptor stretch region of HDO···DOH were observed that allowed us to determine the acceptor switching splitting of Δv = 5.68 cm(-1) in the HDO···DOH vibrational ground state. We suggest that the dominating broadening mechanism is intervibrational relaxation due to coupling of the rovibrational levels of the chromophore via internal droplet excitations.

Single-conformation spectroscopy and population analysis of model γ-peptides: new tests of amide stacking.

Single-conformation ultraviolet and infrared spectra of a series of model gamma-peptides are reported, with the goal of providing new tests of amide stacking as an amide-amide binding motif. The data also serve to illustrate the power and challenges of carrying out single-conformation spectroscopy of neutral molecules of this size in the gas phase under jet-cooled conditions. Building on recent work on Ac-γ2-hPhe-NHMe (James et al., J. Am. Chem. Soc., 2009, 131, 14243), the effects of derivatization and H2O complexation on amide stacking are studied. Ac-γ2-hPhe-N(Me)2 shows only amide stacked structures, blocking the competing position for formation of an amide-amide H-bond. The Ac-γ2-hPhe-NHMe-H2O complex includes structures in which the H2O molecule forms a bridge between the two stacked amide planes, retaining and enhancing amide stacking. IR population transfer methods are also employed to study the dynamics of photodissociation of the amide stacked-H2O complex. Finally, IR ion-gain spectroscopy is introduced as a means of recording infrared spectra containing contributions from all conformers present, based on IR-induced broadening of the UV absorptions. Its role in estimating fractional abundances is tested on Ac-γ2-hPhe-NHMe.

High resolution IR spectroscopy of HDO and HDO(N(2))(n) in helium nanodroplets.

We report on the IR-spectra of HDO and aggregates of HDO and N(2) embedded in superfluid helium nanodroplets in the region of the bound O-D stretch vibration. The R(0) transition 0(00)→1(01) of HDO was observed at 2738.157 50(7)cm(-1). The linewidth of this transition was determined to be 0.060(4)cm(-1). In HDO(N(2))(n) aggregates, the bound O-D stretch vibration was observed at 2732.36(1) and 2730.62(1)cm(-1) for n=1 and n=2, respectively. The line broadening amounts to 1.86(7) and 2.73(7)cm(-1), which correspond to lifetimes of 2.84(1) and 1.94(5) ps.

Aggregation-induced dissociation of HCl(H2O)4 below 1 K: the smallest droplet of acid.

Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl- ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.

High resolution infrared spectroscopy of the asymmetric C-H stretch of 1,2,4,5-tetracyanobenzene (TCNB) and (TCNB)(2) in superfluid helium nanodroplets.

We report the observation of the C-H stretch vibration of 1,2,4,5-tetracyanobenzene (TCNB) and (TCNB)(2) embedded in superfluid helium droplets. The asymmetric C-H stretch of TCNB was observed at 3053.488(3) cm(-1). The corresponding band of the (TCNB)(2) is slightly blueshifted and was observed at 3054.651(2) cm(-1). The intensity of the IR band is increased compared to the monomer absorption. Based on a comparison of our experimental results with predicted IR bands, we conclude that (TCNB)(2) is formed in a C(2h) structure in the droplet.