Yikui Du

Communication: Probing the entrance channels of the X + CH4 → HX + CH3 (X = F, Cl, Br, I) reactions via photodetachment of X−–CH4

The entrance channel potentials of the prototypical polyatomic reaction family X + CH(4) → HX + CH(3) (X = F, Cl, Br, I) are investigated using anion photoelectron spectroscopy and high-level ab initio electronic structure computations. The pre-reactive van der Waals (vdW) wells of these reactions are probed for X = Cl, Br, I by photodetachment spectra of the corresponding X(-)-CH(4) anion complex. For F-CH(4), a spin-orbit splitting (∼1310 cm(-1)) much larger than that of the F atom (404 cm(-1)) was observed, in good agreement with theory. This showed that in the case of the F-CH(4) system the vertical transition from the anion ground state to the neutral potentials accesses a region between the vdW valley and transition state of the early-barrier F + CH(4) reaction. The doublet splittings observed in the other halogen complexes are close to the isolated atomic spin-orbit splittings, also in agreement with theory.

Vibrationally resolved photofragment translational spectroscopy of CH3I from 277 to 304 nm with increasing effect of the hot band.

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

Photofragment translational spectroscopy of n-C3H7I and i-C3H7I near 280 and 304 nm.

The photodissociation dynamics of propyl iodides n-C3H7I and i-C3H7I near 280 and 304 nm has been investigated with our mini-TOF photofragment translational spectrometer. When a single laser is applied for both the photodissociation of parent molecules and the REMPI of I atom photofragments, the TOF spectra of photofragments I*(2P1/2) and I (2P3/2) are obtained at four different wavelengths for these two iodides. For n-C3H7I, some small vibrational peaks are partially resolved (with separation of approximately 522 cm-1, corresponding to the RCH2 deformation frequency of the fragment n-C3H7) at 281.73, 279.71, and 304.67 nm. These results show that the RCH2 deformation is mostly excited. For i-C3H7I, we obtain some partially resolved vibrational peaks (with separation of approximately 352 cm-1, corresponding to the HC(CH3)2 out-of-plane bending frequency of the fragment i-C3H7) at 281.73 nm only. For n-C3H7I, the partitioning values of the available energy Eint/Eavl are 0.48 at 281.73 nm and 0.49 at 304.02 nm for the I* channel, and 0.52 at both 279.71 and 304.67 nm for the I channel. These energy partitioning values are comparable with the previous results at different wavelengths in the literature. For i-C3H7I, the Eint/Eavl values are 0.61 at 281.73 nm, 0.65 at 304.02 nm for the I* channel, and 0.62 at 279.71 nm, 0.49 at 304.67 nm for the I channel. The potential-energy-surface crossing and the beta values have also been discussed.

Photofragment translational spectroscopy of CH3I at 225 nm—with the high excitation of the symmetric stretch vibration of CH3 fragment

The photodissociation dynamics of CH(3)I at 225 nm is studied on our high resolution mini-TOF photofragment translational spectrometer. The photofragment translational spectra of the I* and the I channels via parallel (∥) and perpendicular (⊥) transitions, i.e., of the four pathways (3)Q(0), (3)Q(0) ← (1)Q(1), (1)Q(1), and (1)Q(1) ← (3)Q(0), are obtained with both the symmetric stretch (ν(1)) and the umbrella (ν(2)) vibrational modes of the CH(3) fragments partially resolved. The strong excitation of the symmetric stretch mode (ν(1)) is revealed in both the I and the I* channels. The branching fractions for the four pathways (0.09 for (3)Q(0), 0.03 for (3)Q(0) ← (1)Q(1), 0.34 for (1)Q(1), and 0.54 for (1)Q(1) ← (3)Q(0)) show that the parallel transition ((3)Q(0) ← X) is the major and the I channel is dominant in the photodissociation of CH(3)I at 225 nm. The curve-crossing probability is found to be 0.86 for (1)Q(1) ← (3)Q(0) but 0.08 for (3)Q(0) ← (1)Q(1).

Theoretical and REMPI spectroscopic study on phenylhydrazine and phenylhydrazine–(Ar)n (n = 1, 2) van der Waals complexes

Phenylhydrazine and its van der Waals complexes with one or two argon atoms were investigated with theoretical calculations and resonant two photon ionization (R2PI) spectroscopy. The ab initio and DFT calculations found a conversion of the orbital hybridization of the Nbeta atom from sp3-like in the S0 state to sp2-like in the S1 state, suggesting that the lone pair electrons of the Nbeta atom are involved in a super p-p-pi conjugation over the skeleton of phenylhydrazine in the S1 state. The structural change of the hydrazino group in the S1<--S0 electronic transition was reflected by the vibrational excitations of the hydrazino group observed in the 1C-R2PI spectrum. The band origin of the S1<--S0 transition is determined to be 33610 cm(-1) and the adiabatic ionization energy (IE) of phenylhydrazine, measured by 2C-R2PI spectroscopy, is 62829+/-15 cm(-1). The S1<--S0 electronic transitions of phenylhydrazine-Ar and phenylhydrazine-Ar2 complexes were also observed in the 1C-R2PI spectrum, and their band origins are, respectively, red-shifted by 39 and 80 cm(-1) from that of phenylhydrazine.

Vibrationally mediated photodissociation of CH3I [v1 = 1] at 277.5 nm: the vibrationally adiabatic process.

From the photofragment translational spectra of C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] photodissociatioin at 277.5 nm, the vibrational distribution of photofragments CH3 (v1 = 0, v2 = 0), (0,1), (1,0), (1,1) in the I* channel are measured to be 0.02, 0.02, 0.47, 0.25, and those of CH3 (1,0), (1,1) in the I channel are 0.04, 0.05, respectively. It shows that most of the dissociated CH3I [1,0] retain the C-H symmetric stretch vibration v1 = 1 in the photofragments CH3, and the vibrational distribution in umbrella bending mode is not seriously affected by the original C-H symmetric stretch excitation. The photodissociation of CH3I [1,0] mainly follows the vibrationally adiabatic process. The original vibrational excitation [v1 = 1] of CH3I is quite like a spectator, and the intramolecular vibrational-energy redistribution (IVR) does not play obvious part during photodissociation.

Rotamers and isotopomers of 3-chloro-5-fluoroanisole studied by resonant two-photon ionization spectroscopy and theoretical calculations.

The ab initio and density functional theory (DFT) calculations reveal that two rotamers, denoted by cis and trans 3-chloro-5-fluoroanisole (3C5FA), are stable for each of the S(0), S(1), and D(0) states. In the one-color resonant two-photon ionization (R2PI) spectra, the band origins of the S(1)←S(0) electronic transition (0(0) bands) of cis(35)Cl-3C5FA and cis(37)Cl-3C5FA are both located at 36,468 ± 3 cm(-1), while the 0(0) bands of trans(35)Cl-3C5FA and trans(37)Cl-3C5FA are found to be 36,351 ± 3 and 36,354 ± 3 cm(-1). The two rotamers display very similar vibrational frequencies in the S(1) state, and the observed active modes mainly involve the in-plane ring deformation vibrations. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of both isotopomers of 3C5FA are determined to be 69,720 ± 15 cm(-1) for the cis rotamer and 69,636 ± 15 cm(-1) for the trans rotamer. The substitution, conformation, and isotope effects on the properties of 3C5FA, including the molecular structures, vibrational frequencies, and electronic transition and ionization energies, were also discussed in detail.

Resonance-enhanced two-photon ionization spectroscopy and theoretical calculations of 3,5-difluoroanisole and its Ar-containing complex

The structure and vibrations of 3,5-difluoroanisole (3,5-DFA) in the first electronically excited (S(1)) state were studied by mass-analyzed resonant two-photon ionization (R2PI) technique as well as the quantum chemical calculations. The ab initio and density functional theory (DFT) calculations reveal that only one structure is stable for each of the S(0), S(1), and D(0) states. In the one color R2PI spectrum, the band origin of the S(1)←S(0) electronic transition (0(0) band) of 3,5-DFA is found to be 37,595±3 cm(-1). In the S(1) state, most of the bands observed are related to the in-plane ring deformation and out-of-plane bending vibrations. The adiabatic ionization energy (IE) of 3,5-DFA is determined to be 70,096±15 cm(-1) by the two color R2PI technique, in agreement with the values predicted by the DFT approaches. The dihalogen-substitution effects on the molecular structure, vibrational frequencies, and electronic transition and ionization energies were discussed in detail. The van der Waals complex of 3,5-DFA with argon (3,5-DFA···Ar) was also observed and studied. The 0(0) band of 3,5-DFA···Ar complex is red-shifted by about 9 cm(-1) with respect to that of 3,5-DFA. Both the experimental data and the calculated results indicate that the formation of 3,5-DFA···Ar complex gives only a weak influence on the properties of 3,5-DFA moiety.

Photofragment translational spectroscopy of ICl near 304 and 280 nm: Observation of an intense hot band effect

The photodissociation dynamics of ICl has been studied near 304 and 280 nm on a simple miniature time of flight (mini-TOF) photofragment translational spectrometer with a short pulse of a weak acceleration field. An intense hot band effect was observed. Many small peaks were resolved in each photofragment translational spectrum (PTS). Based on simulations, the principal peaks were assigned not only to the different photodissociation channels (1) I + Cl, (2) I + Cl*, (3) I* + Cl, or (4) I* + Cl*, but also to the different chlorine isotopes (35Cl and 37Cl). Moreover, some extra peaks showed the existence of an intense hot band effect from vibrationally excited ICl molecules, though only a few percent of ICl molecules remained in the vibrationally excited states in our supersonic molecular beam. Based on the spectra near 304 nm, the quantum yield Φ of each channel, the curve crossing, and the branching fraction σ from each transition state were determined.

Sodium cation locations in zeolite omega studied by 23Na VASS NMR

Abstract Overlapping 23 Na NMR signals of zeolite Na-omega are observed by a 2D MAS quadrupolar nutation NMR spectrum and VASS NMR spectra. The low-field Lorentzian shape component centered at − 3 ppm is assigned to the signal of mobile Na + cations located in twelve-membered ring channels. The high-field component, varying considerably with the spinning angles of VASS NMR spectra, is assigned to the signal of less mobile Na + cations located in gmelinite cages. The partition between two categories of Na + cations is determined by the simulation of VASS NMR spectra with the theoretical consideration extended to more general case that both the quadrupolar and chemical shift anisotropic interactions coexist.