UV and IR Absorption Cross-sections of HCHO, HCDO, and DCDO

Comparison among several vibronic coupling methods

A comparison of four approaches to account the vibronic coupling in photoabsorption is performed. The methods considered are nuclear ensemble (NE), direct vibronic coupling (DVC), adiabatic Hessian (AH), and vertical gradient (VG). The case study is the symmetry-forbidden [Formula: see text] [Formula: see text]A[Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text]A[Formula: see text] (n [Formula: see text] [Formula: see text]) transition in formaldehyde. Being forbidden in the equilibrium geometry, this transition is entirely induced by vibronic coupling and constitutes an appropriate case to study the performance of different methods. From DVC, it is found that mode 1 (C=O out-of-plane bending) is the most inducing, followed by mode 6 (in-plane C-H asymmetric stretching) and finally by mode 2 (in-plane C-H asymmetric bending). We were able to correlate 17 out of 20 structures obtained from NE with these modes, showing that these two methods, although different in principle, give comparable results. The simulated spectra were obtained for all methods and compared, and each one has its own advantage. In what concerns the transition studied, NE gives the best description of the spectrum, DVC is the only one that easily gives an absolute value for OOS, and AH and VG are the computationally less expensive methods. From the latter two, VG is the less demanding on computational grounds, since it does not require the excited state Hessian.

Perturbation of the UV transitions of formaldehyde by TiO2 photocatalysts and Aun nanoclusters

In the gas phase, formaldehyde has an electric-dipole forbidden transition that becomes allowed by vibronic coupling. In this paper we explore whether perturbation by surfaces could also enhance light absorption by CH₂O. We investigate the electronic transitions of formaldehyde in the gas phase and interacting with rutile (110) TiO₂, Au_n nanoclusters, and Au_n on (110)-TiO₂. These surfaces are chosen as being representative of metals and metal-oxide minerals, and also because of specific interest in photocatalysts and noble metal nanocluster catalysts. The oscillator strength of the forbidden n → π* transition of formaldehyde in vacuum is investigated by modelling vibrational coupling to the electronic transition with equation-of-motion coupled cluster theory. The excitation energies and oscillator strengths of formaldehyde are calculated for different orientations and distances to the surfaces using the coupled cluster singles and doubles linear response method within the Quantum Mechanical and Molecular Mechanical (QM/MM) model using the aug-cc-pVTZ basis set and compared with the values calculated in vacuo. The electronic transitions of formaldehyde vary very little when placed near a pure TiO₂-surface with only minor variations depending on the orientation of formaldehyde. Introducing a gold nanoparticle (by itself or supported by TiO₂) induces dramatic changes in the absorption properties. This is due to vibronic interactions and the effect of the broken symmetry on the n → π* transition. We see a large redshift in the transition of 90 nm and oscillator strengths larger than 1.0 × 10^-4 for CH₂O interacting with Au_n.

Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence

The effect of the incident UV pump wavelength on the subsequent excited state dynamics, electronic relaxation, and ultimate dissociation of formaldehyde is studied using first principles simulation and Coulomb explosion imaging (CEI) experiments. Transitions in a vibronic progression in the à ← X̃ absorption band are systematically prepared using a tunable UV source which generates pulses centered at 304, 314, 329, and 337 nm. We find, both via ab initio simulation and experimental results, that the rate of excited state decay and subsequent dissociation displays a prominent dependence on which vibronic transition in the absorption band is prepared by the pump. Our simulations predict that nonadiabatic transition rates and dissociation yields will increase by a factor of >100 as the pump wavelength is decreased from 337 to 304 nm. The experimental results and theoretical simulations are in broad agreement and both indicate that the dissociation rate plateaus rapidly after ≈2 ps following an ultrafast sub-ps rise.

Compatibility and thermal decomposition mechanism of nitrocellulose/Cr2O3 nanoparticles studied using DSC and TG-FTIR

Nano metal oxides are common combustion catalysts for enhancing the burning rate of solid propellants. Cr₂O₃ nanoparticles (NPs) are efficient combustion catalysts for the pyrolysis of energetic components. In this study, Cr₂O₃ NPs were synthesized via a modified sol–gel method and further used for studying the thermal decomposition of nitrocellulose (NC). Differential scanning calorimetry (DSC) and thermogravimetry-Fourier-transform infrared spectroscopy (TG-FTIR) analyses indicate that the Cr₂O₃ NPs can be safely used with NC and the mechanism of the reaction between Cr₂O₃/NC and pure NC follows the Avrami–Erofeev equation: f(α) = 3(1 − α)[−ln(1 − α)]^1/3/2. The peak temperature and activation energy (E_a) for the thermal decomposition of Cr₂O₃/NC are lower than those of pure NC. NO₂ was detected at a lower temperature after NC was mixed with Cr₂O₃ NPs; this indicated the catalytically accelerated bond cleavage of NC by Cr₂O₃ NPs.

Cr₂O₃ have good compatibility with nitrocellulose and catalyze decomposition of NC through decrease the activation energy. The mechanism showed Cr₂O₃ can accelerate the O–NO₂ bond cleavage and speed up the secondary reaction.

Computation of the S1 ← S0 Vibronic Absorption Spectrum of Formaldehyde by Variational Gaussian Wavepacket and Semiclassical IVR Methods

The vibronic absorption spectrum of the electric dipole forbidden and vibronically allowed S₁(¹ A₂) ← S₀(¹ A₁) transition of formaldehyde is calculated by Gaussian wavepacket and semiclassical methods, along with numerically exact reference calculations, using the potential energy surface of Fu, Shepler, and Bowman ( J. Am. Chem. Soc. 2011, 133, 7957). Specifically, the variational multiconfigurational Gaussian (vMCG) approach and the Herman-Kluk semiclassical initial value representation (HK-SCIVR) are compared to assess the accuracy and convergence of these methods, benchmarked against numerically exact time-dependent wavepacket propagation (TDWP) on the reference potential energy surface. The vMCG calculation is shown to converge quite well with about 100 variationally evolving Gaussian functions and using a local cubic expansion instead of the conventional local harmonic approximation. By contrast, the HK-SCIVR approach with ∼10⁵ trajectories reproduces the vibrationally structured spectral envelope correctly but yields a strongly broadened spectrum. The comparison of the computed absorption spectrum with experiment shows that the relevant vibronic progressions are reasonably reproduced by all computations, but deviations of the order of 10-100 cm^-1 occur, underscoring that both electronic structure calculations and dynamical approaches remain challenging in the calculation of typical small-molecule excited-state spectra by trajectory-based methods.

Tropospheric photolysis rates of the acetaldehyde isotopologues CD₃CHO and CD₃CDO relative to CH₃CHO measured at the European Photoreactor Facility

Acetaldehyde is a hazardous pollutant found in indoor and ambient air. Acetaldehyde photolysis is pressure- and wavelength-dependent with three distinct product channels. In this study, the photolysis rates of CH3CHO, CD3CDO, and CD3CHO are studied in natural tropospheric conditions using long path FTIR spectroscopy, at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. The average relative photolysis rate as an average of four experiments for the fully deuterated isotopologue is j(CH3CHO)/j(CD3CDO) = 1.75 ± 0.04, and as a result of a single experiment j(CH3CHO)/j(CD3CHO) = 1.10 ± 0.10. These results, combined with our previous determination of j(CH3CHO)/j(CH3CDO) = 1.26 ± 0.03, provide mechanistic insight into the photodissociation dynamics of the photoexcited species. Despite the extensive isotopic scrambling in photoexcited acetaldehyde that has recently been reported, the position of the substitution has a clear effect on the relative photolysis rates.

Assessment of mode-mixing and Herzberg-Teller effects on two-photon absorption and resonance hyper-Raman spectra from a time-dependent approach

A time-dependent approach is presented to simulate the two-photon absorption (TPA) and resonance hyper-Raman scattering (RHRS) spectra including Duschinsky rotation (mode-mixing) and Herzberg-Teller (HT) vibronic coupling effects. The computational obstacles for the excited-state geometries, vibrational frequencies, and nuclear derivatives of transition dipole moments, which enter the expressions of TPA and RHRS cross sections, are further overcome by the recently developed analytical excited-state energy derivative approaches in the framework of time-dependent density functional theory. The excited-state potential curvatures are evaluated at different levels of approximation to inspect the effects of frequency differences, mode-mixing and HT on TPA and RHRS spectra. Two types of molecules, one with high symmetry (formaldehyde, p-difluorobenzene, and benzotrifluoride) and the other with non-centrosymmetry (cis-hydroxybenzylidene-2,3-dimethylimidazolinone in the deprotonated anion state (HDBI(-))), are used as test systems. The calculated results reveal that it is crucial to adopt the exact excited-state potential curvatures in the calculations of TPA and RHRS spectra even for the high-symmetric molecules, and that the vertical gradient approximation leads to a large deviation. Furthermore, it is found that the HT contribution is evident in the TPA and RHRS spectra of HDBI(-) although its one- and two-photon transitions are strongly allowed, and its effect results in an obvious blueshift of the TPA maximum with respect to the one-photon absorption maximum. With the HT and solvent effects getting involved, the simulated blueshift of 1291 cm(-1) agrees well with the experimental measurement.

A new device for formaldehyde and total aldehydes real-time monitoring

A new sensitive technique for the quantification of formaldehyde (HCHO) and total aldehydes has been developed in order to monitor these compounds, which are known to be involved in air quality issues and to have health impacts. Our approach is based on a colorimetric method where aldehydes are initially stripped from the air into a scrubbing solution by means of a turning coil sampler tube and then derivatised with 3-methylbenzothiazolinone-2-hydrazone in acid media (pH = -0.5). Hence, colourless aldehydes are transformed into blue dyes that are detected by UV-visible spectroscopy at 630 nm. Liquid core waveguide LCW Teflon® AF-2400 tube was used as innovative optical cells providing a HCHO detection limit of 4 pptv for 100 cm optical path with a time resolution of 15 min. This instrument showed good correlation with commonly used techniques for aldehydes analysis such as DNPH derivatisation chromatographic techniques with off-line and on-line samplers, and DOAS techniques (with deviation below 6%) for both indoor and outdoor conditions. This instrument is associated with simplicity and low cost, which is a prerequisite for indoor monitoring.

High-resolution absorption studies of the A(1)A2-X(1)A1 2(0)(2)4(0)(1) band of formaldehyde

Absolute peak absorption cross sections and pressure broadening coefficients have been recorded with sub-Doppler limited instrumental resolution for selected rotational lines in the 2(0)(2)4(0)(1) vibronic band of the formaldehyde A(1)A2-X(1)A1 electronic transition. The measured absorption cross sections range between (0.18 +/- 0.01) and (10.1 +/- 0.08) x 10(-19) cm2 molecule(-1) and are considerably larger than values from the literature recorded using apparatus where instrumental broadening was significant. However, comparisons with spectral simulations with equivalent resolution from Smith et al. (J. Phys. Chem. A 2006, 110, 11645-11653) are in excellent agreement. Pressure broadening was studied for the collision partners CH2O, CO2, N2, O2, Ne, Kr, Ar, and He, and the resulting broadening coefficients were found to be reduced in comparison to equivalent values measured in infrared regions, consistent with the reduced dipole moment of the upper state probed in this work. Cavity-enhanced absorption spectroscopy (CEAS) measurements were undertaken using calibrated low concentration (2.9-4.6 ppmv) samples from a permeation source and demonstrate a noise equivalent absorption of 1.2 x 10(-6) cm(-1) Hz(-1/2). This implies a minimum detectable formaldehyde concentration with the current system in atmospheric air of 172 ppbv Hz(-1/2).