UV photoreaction pathways of acetylacetaldehyde trapped in cryogenic matrices.

The broadband UV photochemistry kinetics of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been followed by FTIR and UV spectroscopy. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From previous UV irradiation effects, we have already identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting; even fragmentation seems to be most unlikely due to cryogenic cages confinement. Based on these findings, we have attempted an approach to understand the reaction path of electronic relaxation. Indeed, we have demonstrated, in previous studies, that in the case of malonaldehyde, this electronic relaxation pathway proceeds through singlet states while it proceeds through triplet ones in the case of acetylacetone. We observed CO and CO2 formations when photochemistry is almost observed among nonchelated forms, i.e., when the parent molecule is almost totally consumed. In order to identify a triplet state transition, we have tried to observe a "heavy atom effect" by increasing the weight of the matrix gas, from Ne to Xe, and to quench the T1 state by doping the matrices with O2. It appears that, as in the case of acetylacetone, it is the nonchelated forms that fragment. It also appears that these fragmentations certainly take place in the T1 triplet state and originate in an Π* ← n transition.

UV Photochemistry of Acetylacetaldehyde Trapped in Cryogenic Matrices.

The broad band UV photochemistry of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been studied by IRTF spectroscopy. These experimental results have been supported by B3LYP/6-311G++(2d,2p) calculations in order to get S0 minima together with their harmonic frequencies. On those minima, we have also calculated their vibrationally resolved UV absorption spectra at the time-dependent DFT ωB97XD/6-311++G(2d,2p) level. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From UV irradiation effects we have identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting, even if this last phenomenon seems to be most unlikely due to cryogenic cages confinement. On the basis of these findings, we have attempted a first approach to the reaction path of electronic relaxation. It appeared that, as with acetylacetone, the path of electronic relaxation seems to involve triplet states.

UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices.

UV and IR photochemistries of malonaldehyde, the simplest molecule exhibiting an intramolecular proton exchange, have been studied in four cryogenic matrices at 4.3 K, N2, Ne, Ar, and Xe. Samples have been irradiated using a UV and IR OPO type tunable laser, and with a broad band UV mercury lamp. UV and IR spectra have been recorded and compared with theoretical calculations carried out at the SAC-CI/6-31++G(d,p) (UV transitions) and B3LYP/6-311++G(2d,2p) (IR spectra) levels of theory. After deposition, the intramolecularly H-bonded form is found exclusively, while several open forms are formed upon UV irradiation. These open forms show ability to interconvert upon UV irradiation too. Some of them are also able to isomerize upon selective IR irradiations. The whole set of results allowed us to identify seven isomers among the eight postulated. The photodynamics of the electronic relaxation of malonaldehyde have also been investigated. By following the decay or rise of suited specific vibrational bands in the IR spectra, and by comparing the results with an earlier study of the homologous acetylacetone, we deduced that the electronic relaxation of malonaldehyde proceeds through singlet states, most probably through a 3-fold conical intersection, as postulated from theoretical calculations. In contrast with acetylacetone, malonaldehyde does not show fragmentation after UV excitation.

Vibrational modes of aminothiophenol: a TERS and DFT study.

We report Tip Enhanced Raman Spectroscopy (TERS) mapping and Density Functional (DFT) calculations of aminothiophenol (ATP) grafted on a gold surface. The TERS mapping has demonstrated Raman modes of (ATP) and its dimerised derivative Dimercaptoazobenzene (DMAB). This feature confirms that the plasmon activated chemical reaction of ATP has occurred during TERS measurements. In some specific part of the samples some unidentified Raman modes are observed. We suggest that they could come from intermediate species formed during the conversion of ATP into DMAB. These modes are compared with calculated Raman spectra of some possible intermediate species. These results confirm the high potentiality of TERS measurements for nanochemistry.

Infrared photoisomerization of 1-propanol CD3 and OD trapped in four cryogenic matrices: Ne, N2, Ar, and Xe.

The conformational equilibria and isomerization processes of 1-propanol -OD and -CD3 have been studied by vibrational spectroscopy at low temperatures in four cryogenic matrices to investigate the effect of deuteration on their photochemistry. These isotopic species were selectively irradiated in the νOH and νOD domains, resulting in the identification of several conformers that are able to interconvert upon selective IR irradiation. The experimental results were compared with theoretical geometries obtained at the B3LYP/6-311+G(d) level of theory. Alkyl chain isomerization can be induced in rare gas and nitrogen cryogenic matrices by suitable selective irradiation. Selective excitation of the OH and OD stretches of two Gauche isomers transfers the alkyl chain from the gauche to the trans form. The competition between intramolecular vibrational energy relaxation and the matrix-dopant interaction determines the torsional subspace dynamics of the vibrationally excited propanol molecules.

Unveiling the Surface Structure of Amorphous Solid Water via Selective Infrared Irradiation of OH Stretching Modes.

In the quest to understand the formation of the building blocks of life, amorphous solid water (ASW) is one of the most widely studied molecular systems. Indeed, ASW is ubiquitous in the cold interstellar medium (ISM), where ASW-coated dust grains provide a catalytic surface for solid phase chemistry, and is believed to be present in the Earth's atmosphere at high altitudes. It has been shown that the ice surface adsorbs small molecules such as CO, N2, or CH4, most likely at OH groups dangling from the surface. Our study presents completely new insights concerning the behavior of ASW upon selective infrared (IR) irradiation of its dangling modes. When irradiated, these surface H2O molecules reorganize, predominantly forming a stabilized monomer-like water mode on the ice surface. We show that we systematically provoke "hole-burning" effects (or net loss of oscillators) at the wavelength of irradiation and reproduce the same absorbed water monomer on the ASW surface. Our study suggests that all dangling modes share one common channel of vibrational relaxation; the ice remains amorphous but with a reduced range of binding sites, and thus an altered catalytic capacity.

CH stretching vibration of N-methylformamide as a sensitive probe of its complexation: infrared matrix isolation and computational study.

The complexes between trans-N-methylformamide (t-NMF) and Ar, N(2), CO, H(2)O have been studied by infrared matrix isolation spectroscopy and/or ab initio calculations. The infrared spectra of NMF/Ne, NMF/Ar and NMF/N(2)(CO,H(2)O)/Ar matrices have been measured and the effect of the complexation on the perturbation of t-NMF frequencies was analyzed. The geometries of the complexes formed between t-NMF and Ar, N(2), CO and H(2)O were optimized in two steps at the MP2/6-311++G(2d,2p) level of theory. The four structures, found for every system at this level, were reoptimized on the CP-corrected potential energy surface; both normal and CP corrected harmonic frequencies and intensities were calculated. For every optimized structure the interaction energy was partitioned according to the SAPT scheme and the topological distribution of the charge density (AIM theory) was performed. The analysis of the experimental and theoretical results indicates that the t-NMF-N(2) and CO complexes present in the matrices are stabilized by very weak N-H···N and N-H···C hydrogen bonds in which the N-H group of t-NMF serves as a proton donor. In turn, the t-NMF-H(2)O complex present in the matrix is stabilized by O-H···O(C) hydrogen bonding in which the carbonyl group of t-NMF acts as a proton acceptor. Both, the theoretical and experimental results indicate that involvement of the NH group of t-NMF in formation of very weak hydrogen bonds with the N(2) or CO molecules leads to a clearly noticeable red shift of the CH stretching wavenumber whereas engagement of the CO group as a proton acceptor triggers a blue shift of this wavenumber.

New insights into the photodynamics of acetylacetone: isomerization and fragmentation in low-temperature matrixes.

UV and IR photoreactivities of acetylacetone isolated at 4.3 K in four matrixes (N(2), Ne, Ar, Xe), pure and doped with O(2) are investigated, using either tunable UV and IR optical parametric oscillators, or a broad band mercury lamp. Samples are probed by UV and FTIR spectroscopies: electronic and vibrational transitions are assigned and irradiation kinetics are analyzed. Contrary to what is observed in the gas phase, stereoisomerization is the main reaction observed: UV irradiation breaks the strong H-bond of the stable enolic form of acetylacetone, leading to the observation of non-chelated forms. Isomerization among the different non-chelated forms as well as back-isomerization to the chelated form are also observed under UV irradiation. Similar reactions and reaction rates are observed for the four matrixes, indicating that the inter-system crossing to the T(1) state involved in the isomerization process is very fast, probably due to efficient coupling with phonons, in contrast with gas phase where inter-system crossing is rate-limiting. When matrixes are doped with O(2), dissociation of the non-chelated forms under UV irradiation is observed and fragments, in particular CO, are formed in large amounts. Dissociation through a Norrish type-I reaction is probably one of the reaction channels occurring during electronic relaxation: dissociation is hindered by the surrounding cage in the case of pure matrixes while fragments immediately react with O(2) in the case of doped matrixes. The differences between gas phase and cold solid medium photodynamics of acetylacetone are discussed.

UV and IR photoisomerization of acetylacetone trapped in a nitrogen matrix.

UV- and IR-induced photoisomerization of acetylacetone trapped in a nitrogen matrix at 4.3 K have been carried out using a tunable optical parametric oscillator type laser, or a mercury vapor lamp, coupled with Fourier Transform IR and UV spectroscopies. After deposition, the main form present in the cryogenic matrix is that chelated (enol). Upon UV irradiation, the intramolecular H bond is broken leading to nonchelated isomers among seven possible open forms. These forms have then been irradiated by resonant pi* <-- pi UV irradiation, or by resonant nuOH irradiation. The selective UV irradiation allows us to suggest a first vibrational assignment while the nuOH irradiation leads us to observe interconversions between the nonchelated isomers. In order to support our vibrational assignment, we have carried out theoretical calculations at the B3LYP/6-311++G(2d,2p) level of theory. This study shows that only five isomers are observed among eight postulated.

Proton tunneling estimates for malonaldehyde vibrations from supersonic jet and matrix quenching experiments.

FTIR tunneling splittings for a range of fundamental excitations of malonaldehyde are determined from adiabatic cooling and symmetry breaking experiments in three different molecular states (isolated, coated by Ar layers and embedded in bulk Ar matrices), showing that there is room for improvement in available theoretical models.

The complexes between CH3OH and CF4. Infrared matrix isolation and theoretical studies.

The complex formed between methanol and tetrafluoromethane has been identified in argon and neon matrixes by help of FTIR spectroscopy. Three fundamentals (nu(OH), nu(FCF), and nu(CO)) were observed for the complex isolated in the two matrixes, and the OH stretch was red shifted in a neon matrix and blue shifted in an argon matrix with respect to the corresponding vibration of the methanol monomer. The theoretical studies of the structure and spectral characteristics of the complexes formed between CH(3)OH and CF(4) were carried out at the MP2 level of theory with the 6-311+G(2df,2pd) basis set. The calculations resulted in three stationary points from which two (I-1, I-2) corresponded to structures involving the O-H...F hydrogen bond and the third one (I-3) to the non-hydrogen-bonded structure. The topological analysis of the distribution of the charge density (AIM theory) confirmed the existence of the hydrogen bond in I-1, I-2 complexes and indicated weak interaction between the oxygen atom of CH(3)OH and three fluorine atoms of CF(4) in the I-3 complex. The comparison of the experimental and theoretical data suggests that in the matrixes only the non-hydrogen-bonded complex I-3 is trapped. The blue/red shift of the complex OH stretching vibration with respect to the corresponding vibration of CH(3)OH in argon/neon matrixes is explained by the different sensitivity of the complex and monomer vibrations to matrix material. The ab initio calculations performed for the ternary CH(3)OH-CF(4)-Ar systems indicated a negligible effect of an argon atom on the binary complex frequencies.

Experimental and theoretical UV characterizations of acetylacetone and its isomers.

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.