Nonradiative decay dynamics of methyl-4-hydroxycinnamate and its hydrated complex revealed by picosecond pump-probe spectroscopy

The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H(2)O) in the S(1) state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S(1)-S(0) origin is 8-9 ps. On the other hand, the lifetime of the OMpCA-H(2)O complex at the origin is 930 ps, which is ～100 times longer than that of OMpCA. Furthermore, in the complex the S(1) lifetime shows rapid decrease at an energy of ～200 cm(-1) above the origin and finally becomes as short as 9 ps at ～500 cm(-1). Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA-H(2)O in the S(1) state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S(1) surface, while in the OMpCA-H(2)O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H(2)O is in good agreement with that observed experimentally.     

Novel stereoselective control over cis vs trans opening of benzo[c]phenanthrene 3,4-diol 1,2-epoxides by the exocyclic N(2)-amino group of deoxyguanosine in the presence of hexafluoropropan-2-ol

We describe a novel and efficient synthesis (62-84% yields) of the eight possible, diastereomerically pure, cis and trans, R and S O(6)-allyl-protected N(2)-dGuo phosphoramidite building blocks derived through cis and trans opening of (+/-)-3alpha,4beta-dihydroxy-1beta,2beta-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-1 (1)] and (+/-)-3alpha,4beta-dihydroxy-1alpha,2alpha-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-2 (2)] by hexafluoropropan-2-ol (HFP)-mediated addition of O(6)-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3) at C-1 of the epoxides. Simply changing the relative amount of HFP used in the reaction mixture can achieve a wide ratio of cis/trans addition products. Thus, the observed cis/trans adduct ratio for the reaction of DE-1 (1) in the presence of 5 equiv of 3 varied from 17/83 to 91/9 over the range of 5-532 equiv of HFP. The corresponding ratios for DE-2 (2) varied from 2/98 to 61/39 under the same set of conditions. When 1 or 2 was fused with a 20-fold excess of 3 at 140 degrees C in the absence of solvent HFP, almost exclusive trans addition (>95%) was observed for the both DEs. Through the use of varying amounts of HFP in the reaction mixture as described above, each of the eight possible phosphoramidite oligonucleotide building blocks (DE-1/DE-2, cis/trans, R/S) of the BcPh DE N(2)-dGuo adducts can be prepared in an efficient fashion. To rationalize the varying cis-to-trans ratio, we propose that the addition of 3 to 1 or 2 in the absence of solvent or in the presence of small amounts of HFP proceeds primarily via an S(N)2 mechanism to produce mainly trans-opened adducts. In contrast, increasing amounts of HFP promote increased participation of an S(N)1 mechanism involving a relatively stable carbocation with two possible conformations. One of these conformations reacts with 3 to give mostly trans adduct, while the other conformation reacts with 3 to give mostly cis adduct.     

Femtosecond S3-->S1 conversion and structural reorganisation of trans-3-phenylprop-2-enaldehyde and derivatives in solution.

Photoinduced excited-state relaxation of trans-3-phenylprop-2-enaldehyde (cinnamaldehyde) and three derivatives was studied in hexane and acetonitrile with the pump-supercontinuum-probe technique. Transient spectra were measured with 50 fs resolution in the range 260-660 nm after S3<--S0 excitation at 288 nm. The early spectra reveal an ultrafast, 0.1-0.5 ps, interchange of intensity between two excited-state absorption (ESA) bands followed by a approximately 3 ps decay of ESA. This behaviour is interpreted with the help of semiempirical calculations, which indicate that the sub-picosecond evolution is consistent with S3-->S1 excited-state relaxation while the picosecond decay should be due to a structural intramolecular rearrangement. The latter may consist of a twist of the phenyl ring and rotation around the C==C bond to a perpendicular conformation which corresponds to the global energy minimum in the S1 state and serves as source for trans and cis isomers in the ground state.     

Vibrational energy flow rates for cis- and trans-stilbene isomers in solution

Transient electronic absorption following excitation of the first C-H stretching overtone (2nu(CH)) or a C-H stretch-bend combination (nu(CH) + nu(bend)) monitors the flow of vibrational energy in cis-stilbene and in trans-stilbene. Following a rapid initial rise as energy flows into states interrogated by the probe pulse, the absorption decays with two time constants, which are about a factor of 2 longer for the cis-isomer than for the trans-isomer. The decay times for cis-stilbene are tau2(cis) = (2.6 +/- 1.5) ps and tau3(cis) = (24.1 +/- 2.1) ps, and those for trans-stilbene are tau2(trans) = (1.4 +/- 0.6) ps and tau3(trans) = (10.2 +/- 1.1) ps. The decay times are essentially the same in different solvents, suggesting that the relaxation is primarily intramolecular. The two decay times are consistent with the sequential flow of energy through sets of coupled states within the molecule, and the difference in the rates for the two isomers likely reflects differences in coupling among the states arising from the different structures of the isomers. The similarity of the time evolution following excitation of the first C-H overtone at 5990 cm(-1) and the stretch-bend combination at 4650 cm(-1) is consistent with a subset of states, whose structure is similar for the two vibrational excitation energies, controlling the observed flow of energy.     

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis-->trans tunneling rate

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans-->cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm(-1) region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (> or =4400 cm(-1)), the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans-->cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis-->trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans-->cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans-->cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis-->trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.     

A fast photoswitch for minimally perturbed peptides: investigation of the trans-->cis photoisomerization of N-methylthioacetamide

Thio amino acids can be integrated into the backbone of peptides without significantly perturbing their structure. In this contribution we use ultrafast infrared and visible spectroscopy as well as state-of-the-art ab initio computations to investigate the photoisomerization of the trans form of N-methylthioacetamide (NMTAA) as a model conformational photoswitch. Following the S2 excitation of trans-NMTAA in water, the return of the molecule into the trans ground state and the formation of the cis isomer is observed on a dual time scale, with a fast component of 8-9 ps and a slow time constant of approximately 250 ps. On both time scales the probability of isomerization to the cis form is found to be 30-40%, independently of excitation wavelength. Ab initio CASPT2//CASSCF photochemical reaction path calculations indicate that, in vacuo, the trans-->cis isomerization event takes place on the S1 and/or T1 triplet potential energy surfaces and is controlled by very small energy barriers, in agreement with the experimentally observed picosecond time scale. Furthermore, the calculations identify one S2/S1 and four nearly isoenergetic S1/S0 conical intersection decay channels. In line with the observed isomerization probability, only one of the S1/S0 conical intersections yields the cis conformation upon S1-->S0 decay. A substantially equivalent excited-state relaxation results from four T1/S0 intersystem crossing points.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

The nature of [PdCl(2)(C(2)H(4))(H(2)O)] as an active species in the Wacker process: new insights from ab initio molecular dynamics simulations

First-principles molecular dynamics coupled with metadynamics have been used to gain a deeper insight into the reaction mechanism of the Wacker process by determining the nature of the active species. An explicit and dynamic representation of the aqueous solvent, which was essential for modeling this reaction, was efficiently included into the simulations. Prompted by our earlier results, which showed that the configuration of the catalytically active species [PdCl(2)(H(2)O)(C(2)H(4))] was crucial in the subsequent steps of the Wacker process, herein we focused on the preceding equilibria that led to the formation of both the cis and trans isomers. Starting from the initial catalyst, [PdCl(4)](2-), the free-energy barriers for the forward and backward reactions were calculated. These results confirmed the relevance of the trans intermediate in the reaction mechanism, whilst conversely, they showed that the cis configuration played no role in it. This sole participation of the trans intermediate has some very important implications; besides the mechanistic interpretation of the initial steps in the Wacker reaction mechanism, the analysis of these equilibria provides additional information about the chemical nature of these ligand-substitution processes.     

2-氯-2-氧代(硫代)-1, 3, 2-二氧磷杂环己烷的甲醇解及其开环 异构化反应

报道了trans-2-氧代-2-氯-4-苯基-5,5-二甲基-1,3,2-二氧磷杂环己烷以及cis-2-硫代-2-氯-4-苯基-5,5-二甲基-1,3,2-二氧磷杂环己烷甲醇解反应的立体化学。结果表明,反应体系的酸碱性对前者甲醇解反应的立体化学有着重要影响。而硫代环磷酰氯在碱性条件下的甲醇解反应存在一个开环异构化过程。     

Time-resolved IR spectroscopy of N-methylthioacetamide: trans-->cis isomerization upon n-pi and pi-pi excitation and cis-->trans photoreaction

Time-resolved infrared spectroscopy was used to study the photoisomerization of N-Methylthioacetamide (NMTAA) in D2O in both the cis-->trans and the trans-->cis direction upon selective excitation of the n-pi (S1) and pi-pi (S2) electronic transitions. While isomerization and the return to the ground state takes place on two distinct time scales (cis isomerization is 30-40%, independent of the electronic state excited, while the cis-->trans isomerization proceeds with a 60-70% quantum efficiency. These results support a mechanism by which isomerization takes place via one common intermediate state independent of electronic excitation energy and initial conformation.     

Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.     

Selection of the cis and trans phosph(III)azane macrocycles [{P(mu-NtBu)}2(1-Y-2-NH-C6H4)]2(Y=O, S)

The 1 : 1 reactions of [ClP(mu-NtBu)]2 with the difunctional aromatic amines 1,2-1-YH-2-NH2-C6H4 in the presence of Et3N give the dimeric phosph(III)azane macrocycles [{P(mu-NtBu)2(1-Y-2-HN-C6H4)]2, predominantly as the cis isomer in the case of Y=O (1.cis) and as the trans isomer for Y=S (2.trans). Model M.O. calculations suggest that the selection of the cis and trans isomers is not thermodynamically controlled. The alternative isomers 1.trans and 2.cis are generated exclusively by the deprotonation of the model intermediates [(1-Y-2-NH2-C6H4)P(mu-NtBu)]2[Y=O (3), S (4)] with nBuLi followed by cyclisation with [ClP(mu-NtBu)]2. The solid-state structures of 1.cis/trans(50 : 50), 2.cis, 3 and 4 are reported.     

Resonance Raman Structural Evidence that the Cis-to-Trans Isomerization in Rhodopsin Occurs in Femtoseconds

Picosecond time-resolved resonance Raman spectroscopy is used to probe the structural changes of rhodopsin's retinal chromophore as the cis-to-trans isomerization reaction occurs that initiates vision. Room-temperature resonance Raman spectra of rhodopsin's photoproduct with time delays from -0.7 to 20.8 ps are measured using 2.2 ps, 480 nm pump and 1.5 ps, 600 nm probe pulses. Hydrogen-out-of-plane (HOOP) modes at 852, 871, and 919 cm(-1), fingerprint peaks at 1272, 1236, 1211, and 1166 cm(-1), and a broad red-shifted ethylenic band at 1530 cm(-1) are present at the earliest positive pump-probe time delay of 0.8 ps, indicating that the chromophore is already in a strained, all-trans configuration. Kinetic analyses of both the HOOP and ethylenic regions of the photoproduct spectra reveal that these features grow in with fast ( approximately 200 fs) and slow ( approximately 2-3 ps) components. These data provide the first structural evidence that photorhodopsin has a thermally unrelaxed, torsionally strained all-trans chromophore within approximately 1 ps, and possibly within 200 fs, of photon absorption. Following this ultrafast product formation, the all-trans chromophore cools and conformationally relaxes within a few picoseconds to form bathorhodopsin. This cooling process is revealed as an ethylenic frequency blue-shift of 6 cm(-1) (tau approximately 3.5 ps) as well as an ethylenic width narrowing (tau approximately 2 ps). The ultrafast production of photorhodopsin is likely accompanied by an impulsively driven, localized protein response. More delocalized protein modes are unable to relax on this ultrafast time scale enabling the chromophore-protein complex to store the large amounts of photon energy (30-35 kcal/mol) that are subsequently used to drive activating protein conformational changes.     

Femtosecond Dynamics of Dioxygen - Picket-Fence Cobalt Porphyrins: Ultrafast Release of O(2) and the Nature of Dative Bonding

The ultrafast release of O(2) from the O(2) adduct of picket-fence cobalt porphyrin (see picture) has been probed in real time, and has a total reaction time of 2 ps, without subsequent recombination over several nanoseconds. The dynamics of this ultrafast release of O(2) shows that relaxation within the porphyrin system (200 fs) precedes porphyrin-to-metal electron transfer, but the latter occurs at an enhanced rate (500 fs as opposed to the more usual 1 - 2 ps) because of the dative bonding of cobalt and O(2), which gives the adduct ground state significant Co(III)-O(2)(-) character.     

Fluorinated alcohol mediated displacement of the C10 acetoxy group of benzo[a]pyrene-7,8,9,10-tetrahydrotetraol tetraacetates: a new route to diol epoxide-deoxyguanosine adducts

We describe a novel trifluoroethanol (TFE) or hexafluoropropan-2-ol (HFP) mediated substitution reaction of the bay-region C10 acetoxy group in four stereoisomeric 7,8,9,10-tetraacetoxy-7,8,9,10-tetrahydrobenzo[a]pyrenes (tetraol tetraacetates, two pairs of cis and trans isomers at the 9,10 positions) by the exocyclic N2-amino group of O6-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3). The tetraacetates are derived from cis and trans hydrolysis of (+/-)-7beta,8alpha-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P DE-1) and of (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P DE-2) at C-10 followed by acetylation. Excellent yields and high regioselectivity were observed. Similar cis/trans product ratios were observed for each set of cis and trans tetraol tetraacetates derived from DE-1 ( approximately 75/25) and from DE-2 (approximately 67/33) in HFP. This strongly suggests that the substitution proceeds via an SN1 mechanism involving a carbocation intermediate that is common to the cis and trans tetraacetates. Since it is likely that the cis and trans products from 3 arise from different conformations of the carbocation, its lifetime must be sufficiently long to permit conformational equilibration before its capture by the purine nucleophile. The corresponding reaction of (+/-)-9alpha-bromo-7beta,8alpha,10beta-triacetoxy-7,8,9,10-tetrahydrobenzo[a]pyrene with 3 in HFP was highly regio- and stereoselective and gave exclusively trans 10beta-adducts. This newly developed substitution reaction provides an attractive alternative synthetic strategy for the preparation of polycyclic hydrocarbon adducted oligonucleotide building blocks.     

Synthesis of 7-azabicyclo[2.2.1]heptane and 2-oxa-4-azabicyclo[3.3.1]non-3-ene derivatives by base-promoted heterocyclization of alkyl N-(cis(trans)-3,trans(cis)-4-dibromocyclohex-1-yl)carbamates and N-(cis(trans)-3,trans(cis)-4-dibromocyclohex-1-yl)-2,2,2-trifluoroacetamides

We have studied the base-promoted heterocyclization of alkyl N-(cis(trans)-3,trans(cis)-4-dibromocyclohex-1-yl)carbamates and N-(cis(trans)-3,trans(cis)-4-dibromocyclohex-1-yl)-2,2,2-trifluoroacetamides, investigating the effect of the nitrogen protecting group and the relative configuration of the leaving group at C3 and C4 on the outcome of this reaction. We have observed that the sodium hydride-promoted heterocyclization of alkyl N-(cis-3,trans-4-dibromocyclohex-1-yl)carbamates (10, 12, 14, 16, 18) is a convenient method for the synthesis of 7-azabicyclo[2.2.1]heptane derivatives. For instance, the reaction of tert-butyl N-(cis-3,trans-4-dibromocyclohex-1-yl)carbamate (10) with sodium hydride in DMF at room temperature provides 2-bromo-7-[(tert-butoxy)carbonyl]-7-azabicyclo[2.2.1]heptane (2) (52% yield), whose t-BuOK-promoted hydrogen bromide elimination affords 7-[(tert-butoxy)carbonyl]-7-azabicyclo[2.2.1]hept-2-ene (31) in 78% yield, an intermediate in the total synthesis of epibatidine (1). However, the NaH/DMF-mediated heterocyclization of alkyl N-(trans-3,cis-4-dibromocyclohex-1-yl)carbamates (11, 13) is a more structure dependent reaction, where the nucleophilic attack of the oxygen atom of the protecting group controls the outcome of the reaction, giving rise to benzooxazolone and 2-oxa-4-azabicyclo[3.3.1]non-3-ene derivatives, respectively, from low to moderate yields, in complex reaction mixtures. Conversely, the NaH/DMF heterocyclizations of N-(cis-3,trans-4-dibromocyclohex-1-yl)-2,2,2-trifluoroacetamide (40) or N-(trans-3,cis-4-dibromocyclohex-1-yl)-2,2,2-trifluoroacetamide (42) are very clean reactions giving 7-azabicyclo[2.2.1]heptane or 2-oxa-4-azabicyclo[3.3.1]non-3-ene derivatives, respectively, in good yields. Finally, a mechanistic investigation, based on DFT calculations, has been carried out to rationalize the formation of the different adducts.     

Solvent dependent conformational relaxation of cis-1,3,5-hexatriene

Ultrafast transient absorption spectroscopy was used to study the conformational relaxation dynamics of 1,3,5-cis-hexatriene (Z-HT) produced in the photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) in methanol and n-propanol solvents. The results are compared with earlier investigations performed using cyclohexane and hexadecane solvents [Anderson, N. A.; Pullen, S. H.; Walker II, L. A.; Shiang, J. J.; Sension, R. J.; J. Phys. Chem. A 1998, 102, 10588-10598.]. The conformational relaxation between hot cZc-HT, cZt-HT, and tZt-HT, where the labels c and t designate cis and trans configurations about the single bonds, is much faster in alcohol solvents than in alkane solvents. The hot Z-HT produced in the photochemical ring-opening reaction evolves from the conformationally strained cZc-HT form to the more stable cZt-HT form on a time scale of 2 ps in alcohols compared with 6 ps in alkanes. The overall decay of the internal vibrational temperature of the hot Z-HT is faster in alcohols (5-6 ps) than alkanes (12-20 ps) and is weakly dependent on the specific alcohol or alkane solvent. A small population of cZt-HT (5-10%) is trapped as the solute equilibrates with the surrounding solvent following UV excitation of CHD or direct UV excitation of Z-HT. The influence of solvent on conformational relaxation of Z-HT was investigated further by probing the temperature dependence of the decay of this thermally equilibrated cZt-HT population. The apparent barrier for the cZt --> tZt conformational isomerization is lower in alcohols (17.4 kJ/mol) than in alkanes (23.5 kJ/mol). However the equilibrium Arrhenius prefactor (A(h)) is an order of magnitude smaller for alcohols (ca. 4 x 10(12)) than alkanes (ca. 6 x 10(13)) resulting in an absolute rate of decay that is faster in the alkane than in the alcohol solvents. These results are discussed in the context of transition state theory and Kramers' theory for condensed phase reaction dynamics.     

Stereochemistry of the [4 + 2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C(60)

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C(60) proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C(60), isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C(60), to form adduct 1a. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis,trans-2,4-hexadiene with C(60), which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.     

Ultrafast dynamics and coherent oscillations in ethylene and ethylene-d4 excited at 162 nm

The fifth harmonic (162 nm, 11 fs), generated in a short argon cell from 12 fs Ti-sapphire laser pulses, was used to excite C2H4 and C2D4 in the maximum of the first pi pi* transition. Around 10% of the molecules were excited to the pi3s Rydberg state instead. The subsequent motion of the wave packet, moving over the potentials from the Franck-Condon region down to the ground state, was monitored by nonresonant ionization at 810 nm with mass-selective detection of the ion yield. Five time constants (from approximately 20 fs in excited states to 0.6-11 ps in the hot ground state) and four coherent oscillations (CC stretch and torsion vibrations or hindered free rotation) were determined for each isotopomer. The initial relaxation follows a superposition of CC twist and stretch coordinates; this explains a surprisingly small deuterium isotope effect of the initial time constant (21 versus 24 fs). Also the vibrations in the Franck-Condon region have such a mixed character and a correspondingly small isotope shift. From the perpendicular minimum the wave packet reaches (within 17 or 21 fs for the two isotopomers) a conical intersection via a direction that also involves partial hydrogen migration. This is concluded from the detection of ethylidene (CH3CH), formed simultaneously with ground-state ethylene. This carbene isomerizes in the ground state within 0.6 ps (1.6 ps for CD3CD) to ethylene. Two time constants for dissociation (4.5 and 11 ps) in the hot ground state were also identified. The small yields of bimolecular reactions (photodimerization, addition reactions involving a "suddenly polarized" excited state, carbene reactions) are interpreted in terms of the short lifetimes. It is pointed out that the relaxation path starting from the Rydberg state merges into that from the pi pi* state; nevertheless, there is a wavelength dependence in the photochemistry of olefins, because due to a momentum effect the wave packet remembers from which state it came.     

Femtosecond studies of charge-transfer mediated proton transfer in 2-butylamino-6-methyl-4-nitropyridine N-oxide

We have unraveled the effects of an amino substituent in the ortho position on the excited-state dynamics of 4-nitropyridine N-oxide by studying the picosecond fluorescence kinetics and femtosecond transient absorption of a newly synthesized compound, 2-butylamino-6-methyl-4-nitropyridine N-oxide, and by quantum chemical calculations. Similar to the parent compound, the S(1) state of the target molecule has significant charge-transfer character and shows a large (approximately 8000 cm(-1)) static Stokes shift in acetonitrile. Analysis of the experimental and the theoretical results leads, however, to a new scenario in which this intramolecular charge transfer triggers in polar, aprotic solvents an ultrafast (around 100 fs) intramolecular proton transfer between the amino and the N-O group. The electronically excited N-OH tautomer is subsequently subject to solvent relaxation and decays with a lifetime of approximately 150 ps to the ground state.