Femtosecond dynamics of hydrogen elimination: benzene formation from cyclohexadiene

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time study of the hydrogen elimination reaction of 1,4-cyclohexadiene. The experimental observation of the ultrafast stepwise H-elimination elucidates the reaction dynamics and mechanism. With density-functional theory (ground-state) calculations, the nature of the reaction (multiple) pathways is examined. With the help of recent conical-intersection calculations, the excited-state and ground-state pathways are correlated. From these experimental and theoretical results we provide a unifying picture of the thermochemistry, photochemistry and the stereochemistry observed in the condensed phase

Direct observation of the femtosecond nonradiative dynamics of azulene in a molecular beam: The anomalous behavior in the isolated molecule

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time observation of the nonradiative, anomalous dynamics of azulene. We studied both S_2 and S_1 state dynamics. The motion of the wave packet in S_1 involves two time scales, a dephasing time of less than 100 fs and a 900±100 fs internal conversion. We discuss the dynamical picture in relation to the molecular structures and the conical intersection, and we compare with theory

Direct observation of the femtosecond nonradiative dynamics of azulene in a molecular beam: The anomalous behavior in the isolated molecule

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time observation of the nonradiative, anomalous dynamics of azulene. We studied both S_2 and S_1 state dynamics. The motion of the wave packet in S_1 involves two time scales, a dephasing time of less than 100 fs and a 900±100 fs internal conversion. We discuss the dynamical picture in relation to the molecular structures and the conical intersection, and we compare with theory

Femtochemistry of trans-Azomethane: A Combined Experimental and Theoretical Study

The dissociation dynamics of trans-azomethane upon excitation to the S_1(n,π^*) state with a total energy of 93 kcal mol^(−1) is investigated using femtosecond-resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump–probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one-photon symmetry-forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two-photon symmetry-allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C—N bonds of the molecule break in a stepwise manner, that is, the first C—N bond breaks in ≈70 fs followed by the second one in ≈100 fs, with the intermediate characterized. At the parallel polarization, the first C—N bond cleavage was found to occur in 100 fs with the intensity of the symmetry-allowed transition being one order of magnitude greater than the intensity of the symmetry-forbidden transition at the perpendicular polarization. Theoretical calculations using time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low-lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism: The initial driving force of the ultrafast bond-breaking process of trans-azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity-borrowing mechanism for the symmetry-forbidden n–π^* transition. Following this torsional motion and the associated molecular symmetry breaking, an S_0/S_1conical intersection (CI) can be reached at a torsional angle of 93.1° (predicted at the CASSCF(8,7)/cc-pVDZ level of theory). Funneling through the S_0/S_1CI could activate the asymmetric C—N stretching motion, which is the key motion for the consecutive C—N bond breakages on the femtosecond time scale

Femtochemistry of trans-Azomethane: A Combined Experimental and Theoretical Study

The dissociation dynamics of trans-azomethane upon excitation to the S_1(n,π^*) state with a total energy of 93 kcal mol^(−1) is investigated using femtosecond-resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump–probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one-photon symmetry-forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two-photon symmetry-allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C—N bonds of the molecule break in a stepwise manner, that is, the first C—N bond breaks in ≈70 fs followed by the second one in ≈100 fs, with the intermediate characterized. At the parallel polarization, the first C—N bond cleavage was found to occur in 100 fs with the intensity of the symmetry-allowed transition being one order of magnitude greater than the intensity of the symmetry-forbidden transition at the perpendicular polarization. Theoretical calculations using time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low-lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism: The initial driving force of the ultrafast bond-breaking process of trans-azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity-borrowing mechanism for the symmetry-forbidden n–π^* transition. Following this torsional motion and the associated molecular symmetry breaking, an S_0/S_1conical intersection (CI) can be reached at a torsional angle of 93.1° (predicted at the CASSCF(8,7)/cc-pVDZ level of theory). Funneling through the S_0/S_1CI could activate the asymmetric C—N stretching motion, which is the key motion for the consecutive C—N bond breakages on the femtosecond time scale

Theoretical investigation of the potential energy surface for the NH2+NO reaction via density functional theory and ab initio molecular electronic structure theory

The potential energy surface of the NH+NO reaction, which involves nine intermediates (1-9) as well as twenty-three possible transition states (a-w), has been fully characterized at the B3LYP/cc-pVQZ//B3LYP/6-311G(d,p) + ZPE[B3LYP/6-311G(d,p)] and modified Gaussian-2 (G2M) levels of theory. The reaction is shown to have three different groups of products (HN+OH, NO+H, and N+HO denoted as A, B, and C, respectively) and a very complicated reaction mechanism. The first reaction path is initiated by the N-N bond association of the reactants to form an intermediate HNNO, 1, which then undergoes a 1,3-H migration to yield an isomer pair HNNOH (2,3) (separated by a low energy torsional barrier) which can then proceed along three different paths. Because of the essential role it would play kinetically, the enthalpy of the NH+NO→HN+OH reaction has been further investigated using various levels of theory. The best theoretical results of this study predicted it to be 0.9 and 2.4 kcal mol at the B3LYP and CCSD(T) levels, respectively, using a relatively large basis set (AUG-cc-pVQZ) based on the geometry optimized at the B3LYP/6-311G(d,p) level of theory. It has been found that TS g(4→B) is expected to be the rate-determining transition state responsible for the NH+NO→NO+H reaction. TS g lies above the reactants by only 2.6 kcal mol according to the G2M prediction. On the other hand, TS h(3→7) is a new transition state discovered in this work which may allow some kinetic contribution from the NH+NO→N+HO reaction under high temperature conditions due to its relatively low energy as well as its loose transition state property. A modified G2 additivity scheme based on the G2(DD) approach has been shown to be necessary for better predicting the energetics for TS h, which gives a value of 2.3 kcal mol in energy with respect to the reactants. Generally, the cost-effective B3LYP method is found to give very good predictions for the optimized geometries and vibrational frequencies of various species in the system if compare them with those optimized at the QCISD/6-311G(d,p) and 12-in-11 CASSCF/cc-pVDZ levels of theory. Furthermore, it is noticeable in this study that most of the relative energies calculated via the B3LYP method are more close to the G2M results than those predicted at the PMP4 and CCSD(T) levels using the same 6-311G(d,p) basis set

Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S_1 Surface

The dissociation dynamics of two acetone isotopomers ([D_0]- and [D_6]acetone) after 93 kcal mol^(−1) (307 nm) excitation to the S_1(n,π^*) state have been investigated using femtosecond pump–probe mass spectrometry. We found that the nuclear motions of the molecule on the S_1 surface involve two time scales. The initial femtosecond motion corresponds to the dephasing of the wave packet out of the Franck–Condon region on the S_1 surface. For longer times, the direct observation of the build-up of the acetyl radical confirms that the S_1α-cleavage dynamics of acetone is on the nanosecond time scale. Density functional theory and ab initio calculations have been carried out to characterize the potential energy surfaces for the S_0, S_1, and T_1 states of acetone and six other related aliphatic ketones. For acetone, the S_1 energy barrier along the single α-positioned carbon–carbon (α-CC) bond-dissociation coordinate (to reach the S_0/S_1 conical intersection) was calculated to be 18 kcal mol^(−1) (∼110 kcal mol^(−1) above the S_0 minimum) for the first step of the nonconcerted α-CC bond cleavage; the concerted path is energetically unfavorable, consistent with experiments. The S_1 barrier heights for other aliphatic ketones were found to be substantially lower than that of acetone by methyl substitutions at the α-position. The α-CC bond dissociation energy barrier of acetone on the T_1 surface was calculated to be only 5 kcal mol^(−1) (∼90 kcal mol^(−1) above the S_0 minimum), which is substantially lower than the barrier on the S_1 surface. Based on the calculations, the α-cleavage reaction mechanism of acetone occurring on the S_0, S_1, and T_1 surfaces can be better understood via a simple physical picture within the framework of valence-bond theory. The theoretical calculations support the conclusion that the observed nanosecond-scale S_1 dynamics of acetone below the barrier is governed by a rate-limiting S_1 → T_1 intersystem crossing process followed by α-cleavage on the T_1 surface. However, at high energies, the α-cleavage can proceed by barrier crossing on the S1 surface, a situation which is demonstrated for cyclobutanone in the accompanying paper

Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S_1 Surface

The dissociation dynamics of two acetone isotopomers ([D_0]- and [D_6]acetone) after 93 kcal mol^(−1) (307 nm) excitation to the S_1(n,π^*) state have been investigated using femtosecond pump–probe mass spectrometry. We found that the nuclear motions of the molecule on the S_1 surface involve two time scales. The initial femtosecond motion corresponds to the dephasing of the wave packet out of the Franck–Condon region on the S_1 surface. For longer times, the direct observation of the build-up of the acetyl radical confirms that the S_1α-cleavage dynamics of acetone is on the nanosecond time scale. Density functional theory and ab initio calculations have been carried out to characterize the potential energy surfaces for the S_0, S_1, and T_1 states of acetone and six other related aliphatic ketones. For acetone, the S_1 energy barrier along the single α-positioned carbon–carbon (α-CC) bond-dissociation coordinate (to reach the S_0/S_1 conical intersection) was calculated to be 18 kcal mol^(−1) (∼110 kcal mol^(−1) above the S_0 minimum) for the first step of the nonconcerted α-CC bond cleavage; the concerted path is energetically unfavorable, consistent with experiments. The S_1 barrier heights for other aliphatic ketones were found to be substantially lower than that of acetone by methyl substitutions at the α-position. The α-CC bond dissociation energy barrier of acetone on the T_1 surface was calculated to be only 5 kcal mol^(−1) (∼90 kcal mol^(−1) above the S_0 minimum), which is substantially lower than the barrier on the S_1 surface. Based on the calculations, the α-cleavage reaction mechanism of acetone occurring on the S_0, S_1, and T_1 surfaces can be better understood via a simple physical picture within the framework of valence-bond theory. The theoretical calculations support the conclusion that the observed nanosecond-scale S_1 dynamics of acetone below the barrier is governed by a rate-limiting S_1 → T_1 intersystem crossing process followed by α-cleavage on the T_1 surface. However, at high energies, the α-cleavage can proceed by barrier crossing on the S1 surface, a situation which is demonstrated for cyclobutanone in the accompanying paper

Femtochemistry of Norrish Type-I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S_1 Surface

The anomalous nonradiative dynamics for three cyclobutanone isotopomers ([D_0]-, 3,3-[D_2]-, and 2,2,4,4-[D_4]cyclobutanone) have been investigated using femtosecond (fs) time-resolved mass spectrometry. We have found that the internal motions of the molecules in the S_1 state above the dissociation threshold involve two time scales. The fast motion has a time constant of <50 fs, while the slow motion has a time constant of 5.0±1.0, 9.0±1.5, and 6.8±1.0 ps for the [D_0], [D_2], and [D_4] species, respectively. Density functional theory and ab initio calculations have been performed to characterize the potential energy surfaces for the S_0, S_1(n,π^*), and T_1(n,π^*) states. The dynamic picture for bond breakage is the following: The fast motion represents the rapid dephasing of the initial wave packet out of the Franck–Condon region, whereas the slow motion reflects the α-cleavage dynamics of the Norrish type-I reaction. The redistribution of the internal energy from the initially activated out-of-plane bending modes into the in-plane ring-opening reaction coordinate defines the time scale for intramolecular vibrational energy redistribution (IVR), and the observed picosecond-scale (ps) decay gives the rate of IVR/bond cleavage across the barrier. The observed prominent isotope effect for both [D_2] and [D_4] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S_1α-cleavage dynamics. The ethylene and ketene (C_2 products)—as well as CO and cyclopropane (C_3 products)—product ratios can be understood by the involvement of an S_0/S_1 conical intersection revealed in our calculations. This proposed dynamic picture for the photochemistry of cyclobutanone on the S_1 surface can account not only for the abnormally sharp decrease in fluorescence quantum yield and lifetime but also for the dramatic change in the C_3:C_2 product ratio as a function of increasing excitation energy, as reported by Lee and co-workers (J. C. Hemminger, E. K. C. Lee, J. Chem. Phys.1972, 56, 5284–5295; K. Y. Tang, E. K. C. Lee, J. Phys. Chem.1976, 80, 1833–1836)