On the role of methyl torsional modes in the intersystem crossing dynamics of isolated molecules

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6'=1.56 and V6'=8.28 cm(-1) in 2MP and V6'=4.11 and V6'=58.88 cm(-1) in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the sigma=+/-1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the sigma=0 torsional levels.     

Ultrafast dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine

The dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine (p-NPP) has been investigated using the subpicosecond transient absorption spectroscopic technique in different kinds of solvents. Following photoexcitation using 400 nm light, conformational relaxation via twisting of the nitro group, internal conversion (IC) and the intersystem crossing (ISC) processes have been established to be the three major relaxation pathways responsible for the ultrafast deactivation of the excited singlet (S(1)) state. Although the nitro-twisting process has been observed in all kinds of solvents, the relative probability of the occurrence of the other two processes has been found to be extremely sensitive to solvent polarity, because of alteration of the relative energies of the S(1) and the triplet (T(n)) states. In the solvents of lower polarity, the ISC is predominant over the IC process, because of near isoenergeticity of the S(1)(ππ*) and T(3)(nπ*) states. On the other hand, in the solvents of very large polarity, the energy of the S(1)(ππ*) state becomes lower than those of both the T(3)(nπ*) and T(2)(nπ*/ππ*) states, but those of the T(1)(ππ*) state and the IC process to the ground electronic (S(0)) state are predominant over the ISC, and hence the triplet yield is nearly negligible. However, in the solvents of medium polarity, the S(1) and T(2) states become isoenergetic and the deactivation of the S(1) state is directed to both the IC and ISC channels. In the solvents of low and medium polarity, following the ISC process, the excited states undergo IC, vibrational relaxation, and solvation in the triplet manifold. On the other hand, following the IC process in the Franck-Condon region of the S(0) state, the vibrationally hot molecules with the twisted nitro group subsequently undergo the reverse nitro-twisting process via dissipation of the excess vibrational energy to the solvent or vibrational cooling.     

An experimental and theoretical study of the S(1)<--S0 transition of p-ethynyltoluene

The one photon and the two photon S(1)<--S(0) spectra of jet-cooled p-ethynyltoluene have been measured for the first time, and a detailed vibronic analysis for both spectra has been attained. Mass analyzed resonance enhanced multiphoton ionization spectroscopy is the employed technique. In the one photon spectrum, the allowed component (origin and Franck-Condon bands) is much weaker than the forbidden component, and the same mechanisms as in the one photon spectrum of phenylacetylene are observed. The methyl torsional transitions are active. The 0(0) (0) band is at 35 483 cm(-1). The two photon spectrum is very strong and bears a resemblance to the two photon spectrum of phenylacetylene. The potential barrier of the methyl rotor in the S(1) state has been determined as V(6)=-12 cm(-1) with B(CH(3) )=5.55 cm(-1). Ab initio calculations, MP2(full)/cc-pVTZ and CAS/cc-pVTZ, have been implemented for the geometry optimization and the normal mode vibration computation in the S(0) and S(1) states.     

Stability analysis of the two rotational isomers of meta-fluorobenzaldehyde in the S0, S1, and S2 electronic states

For a determination of the stabilization energy between the two rotational isomers of m-fluorobenzaldehyde, the S(1,2)<--S0 absorption spectra were observed in fluid n-hexane solutions at 293 and 198 K. After employing a simulation method for the spectra, we succeeded in determining the stabilization energies in the ground and the relevant excited state at the same time. The energy was estimated to be 1.7 +/- 0.5 kJ mol(-1) for S0 and also for S1 while it was 2.9 +/- 0.5 kJ mol(-1) for S2. Ab initio calculations at MP2/6-311G** computational level predicted that the O-cis form in the S0 state is more stable by 1.9 +/- 0.5 kJ mol(-1) than the counterpart rotamer, O-trans form. In summary, a schematic energy level diagram of the two rotational isomers will be illustrated for the S0, S1, and S2 states in the fluid system.     

Vibrations and theoretical calculations of p-methylanisole in the first electronically excited S1 and ionic ground D0 states

One-color (1C), two-color (2C) resonant two-photon ionization (R2PI), and mass analyzed threshold ionization (MATI) methods have been applied to study the S(1)<--S(0) transition and threshold ionization of p-methylanisole. The excitation energy of the S(1)<--S(0) transition is determined to be 35,401+/-2 cm(-1), the adiabatic ionization energy of this molecule is measured to be 63,965+/-15 and 63,972+/-5 cm(-1) by the 2C-R2PI and MATI methods. Most of the observed R2PI and MATI bands result from the in-plane ring vibrations. The frequencies of vibrations 9b, 1 and 7a are measured to be 393, 800 and 1168 cm(-1) in the S(1) state, and 412, 811 and 1220 cm(-1) in the D(0) state, respectively. This indicates the molecular structure in the D(0) state is more rigid than that in the S(1) state.     

Low-lying excited states and nonradiative processes of the adenine analogues 7H- and 9H-2-aminopurine

We have investigated the UV vibronic spectra and excited-state nonradiative processes of the 7H- and 9H-tautomers of jet-cooled 2-aminopurine (2AP) and of the 9H-2AP-d(4) and -d(5) isotopomers, using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.045 cm(-1) resolution. The S(1) ← S(0) transition of 7H-2AP was observed for the first time. It lies ～1600 cm(-1) below that of 9H-2AP, is ～1000 times weaker and exhibits only in-plane vibronic excitations. In contrast, the S(1) ← S(0) spectra of 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ν(1)', ν(2)', and ν(3)'. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the (1)ππ?(L(a)) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a "butterfly" deformation in its lowest (1)ππ? state. The rotational contours of the 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) 0(0)(0) bands and of eight vibronic bands of 9H-2AP up to 0(0)(0) + 600 cm(-1) exhibit 75%-80% in-plane (a∕b) polarization, which is characteristic for a (1)ππ? excitation. A 20%-25% c-axis (perpendicular) transition dipole moment component may indicate coupling of the (1)ππ? bright state to the close-lying (1)nπ? dark state. However, no (1)nπ? vibronic bands were detected below or up to 500 cm(-1) above the (1)ππ? 0(0)(0) band. Following (1)ππ? excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime ≥5 μs. The ionization potential of 9H-2AP was measured via the (1)ππ? state (IP = 8.020 eV) and the long-lived state (IP > 9.10 eV). The difference shows that the long-lived state lies ≥1.08 eV below the (1)ππ? state. Time-dependent B3LYP calculations predict the (3)ππ? (T(1)) state 1.12 eV below the (1)ππ? state, but place the (1)nπ? (S(1)) state close to the (1)ππ? state, implying that the long-lived state is the lowest triplet (T(1)) and not the (1)nπ? state.     

Fluorescence and REMPI spectroscopy of jet-cooled isolated 2-phenylindene in the S1 state

We investigated the spectroscopy of the first excited singlet electronic state S1 of 2-phenylindene using both fluorescence excitation spectroscopy and resonantly enhanced multiphoton ionization spectroscopy. Moreover, we investigated the dynamics of the S1 state by determining state-selective fluorescence lifetimes up to an excess energy of approximately 3400 cm(-1). Ab initio calculations were performed on the torsional potential energy curve and the equilibrium and transition state geometries and normal-mode frequencies of the first excited singlet state S1 on the CIS level of theory. Numerous vibronic transitions were assigned, especially those involving the torsional normal mode. The torsional potentials of the ground and first excited electronic states were simulated by matching the observed and calculated torsional frequency spacings in a least-squares fitting procedure. The simulated S1 potential showed very good agreement with the ab initio potential calculated on the CIS/6-31G(d,p) level of theory. TDDFT energy corrections improved the match with the simulated S(1) torsional potential. The latter calculation yielded a torsional barrier of V2 = 6708 cm(-1), and the simulation a barrier of V2 = 6245 cm(-1). Ground-state normal-mode frequencies were calculated on the B3LYP/6-31G(d,p) level of theory, which were used to interpret the infrared spectrum, the FDS spectrum of the transition and hot bands of the FES spectrum. The fluorescence intensities of the nu49 overtone progression could reasonably be reproduced by considering the geometry changes upon electronic excitation predicted by the ab initio calculations. On the basis of the torsional potential calculations, it could be ruled out that the uniform excess energy dependence of the fluorescence lifetimes is linked to the torsional barrier in the excited state. The rotational band contour simulation of the transition yielded rotational constants in close agreement to the ab initio values for both electronic states. Rotational coherence signals were obtained by polarization-analyzed, time-resolved measurements of the fluorescence decay of the transition. The simulation of these signals yielded corroborating evidence as to the quality of the ab initio calculated rotational constants of both states. The origin of the anomalous intensity discrepancy between the fluorescence excitation spectrum and the REMPI spectrum is discussed.     

S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices

The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.     

Twisted intramolecular charge transfer states: rotationally resolved fluorescence excitation spectra of 4,4'-dimethylaminobenzonitrile in a molecular beam

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.     

Phosphorescence excitation spectrum of the T(1)(n,pi*)<--S(0) transition of 4H-pyran-4-one

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40-50 degrees C was recorded near 366 nm. The most intense vibronic feature in this region of the spectrum is the T(1)(n,pi*)<--S(0) origin band. The value of nu(0) for the 0(0)(0) transition was determined to be 27 291.5 cm(-1) by comparing the observed spectrum to a simulation in the T(1)<--S(0) origin-band region. Attached to the origin band in the PE spectrum are several Deltav=0 sequence bands involving low-frequency ring modes. From the positions of these bands, together with the known ground-state combination differences, fundamental frequencies for nu(18') (ring bending), nu(13') (ring twisting), and nu(10') (in-plane ring deformation) in the T(1)(n,pi*) excited state were determined to be 126, 269, and 288 cm(-1), respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S(0) state. The changes in these ring frequencies indicate that the effects of T(1)(n,pi*)<--S(0) excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T(1)(n,pi*) than for S(1)(n,pi*) excitation.     

Microwave and UV excitation spectra of 4-fluorobenzyl alcohol at high resolution. S0 and S1 structures and tunneling motions along the low frequency -CH2OH torsional coordinate in both electronic states

Rotationally resolved electronic spectra of several low frequency vibrational bands that appear in the S(1) ← S(0) transition of 4-fluorobenzyl alcohol (4FBA) in the collision-free environment of a molecular beam have been observed and assigned. Each transition is split into two or more components by the tunneling motion of the attached -CH(2)OH group. A similar splitting is observed in the microwave spectrum of 4FBA. Analyses of these data show that 4FBA has a gauche structure in both electronic states, but that the ground state C(1)C(2)-C(7)O dihedral angle of ～60° changes by ～30° when the photon is absorbed. The barriers to the torsional motion of the attached -CH(2)OH group are also quite different in the two electronic states; V(2) ～ 300 cm(-1) high and ～60° wide in the S(0) state, and V(2) ～ 300 cm(-1) high and ～120° wide (or V(2) ～ 1200 cm(-1) high and ～60° wide) in the S(1) state. Possible reasons for these behaviors are discussed.     

Cavity ring-down spectroscopy and theoretical calculations of the S1(1B3u)<--S0(1Ag) transition of jet-cooled perylene

As part of our long-term program to test the diffuse interstellar band-polycyclic aromatic hydrocarbon hypothesis, we have investigated the S(1)<--S(0) electronic transition of neutral perylene (C(20)H(12)) in a combined experimental and theoretical study. Jet-cooled perylene was prepared with a pulsed discharge slit nozzle and detected by cavity ring-down spectroscopy. A number of vibronic features were observed in the 24 000-24 900 cm(-1) spectral range. Density functional and ab initio calculations were performed to determine the geometries, harmonic vibrational frequencies, and normal coordinates of both the S(0) and S(1) electronic states. A rotational temperature of 52+/-5 K was derived from a rotational contour analysis of the vibronic band associated with the 0-0 transition. A Franck-Condon treatment was carried out to calculate the vibronic spectrum of the S(1)<--S(0) transition. A good agreement was found between the calculated and the experimental spectra. A vibrational assignment is proposed and six normal modes are identified. The contribution of neutral compact polycyclic aromatic hydrocarbons to the diffuse interstellar bands is briefly discussed.     

Structural Control of (1)A(2u)-to-(3)A(2u) Intersystem Crossing in Diplatinum(II,II) Complexes

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt(2)(μ-P(2)O(5)(BF(2))(2))(4)](4-), abbreviated Pt(pop-BF(2))), yields a radiative decay rate (k(r) = 1.7 × 10(8) s(-1)) an order of magnitude greater than that of the parent complex, Pt(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt(pop) [ISC activation energies: 2230 cm(-1) for Pt(pop-BF(2)); 1190 cm(-1) for Pt(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin-orbit coupling between the (1)A(2u) state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E(a) is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt(2)(P-O-P)(4) core that would allow direct (1)A(2u)-(3)A(2u) spin-orbit coupling.     

High-resolution laser spectroscopy on the S1<--S0 transition of jet-cooled anthracene: rotational structure and Stark effect

We present rotationally resolved spectra of the S(1)<--S(0) transition of anthracene at 27,687.153(4) cm(-1) as well as Stark effect measurements of the free anthracene molecule in electric fields of up to 85 kV/cm. The molecule is rotationally cooled in a supersonic jet expansion to a temperature of 4 K. The rotational constants of the electronic states S(0) and S(1) are determined by a simplex fit comparing the experimental spectra with simulations for an asymmetric rigid rotor. The measured and simulated energies are in very good agreement and the estimated accuracy of the rotational constants is 1 per thousand. Furthermore, the polarizabilities of the electronic states S(0) and S(1) are investigated. At an electric field of 85 kV/cm, line shifts of up to 150 MHz caused by a change in the polarizability of Deltaalpha=123(7) a.u. and broadenings due to the anisotropy are observed. The components of the tensor polarizabilities of the electronic states S(0) and S(1) are determined by simulating the complete spectra using second-order perturbation theory.     

Anomalous splittings of torsional sublevels induced by the aldehyde inversion motion in the S1 state of acetaldehyde

The G6 group-theoretical high-barrier formalism developed previously for internally rotating and inverting CH3NHD is used to interpret the abnormal torsional splittings in the S1 state of acetaldehyde for levels 14(0-)15(0), 14(0-)15(1), and 14(0-)15(2), where 14(0-) denotes the upper inversion tunneling component of the aldehyde hydrogen and 15 denotes the methyl torsional vibration. This formalism, derived using an extended permutation-inversion group G6m, treats simultaneously methyl torsional tunneling, aldehyde-hydrogen inversion tunneling and overall rotation. Fits to the rotational states of the four pairs of inversion-torsion vibrational levels (14(0+)15(0A,E), 14(0-)15(0A,E)), (14(0+)15(1A,E), 14(0-)15(1A,E)), (14(0+)15(2A,E), 14(0-)15(2A,E)), and (14(0+)15(3A,E), 14(0-)15(3A,E)) are performed, giving root-mean-square deviations of 0.003, 0.004, 0.004, and 0.004 cm(-1), respectively, which are nearly equal to the experimental uncertainty of 0.003 cm(-1). For torsional levels lying near the top of the torsional barrier, this theoretical model, after including higher-order terms, provides satisfactory fits to the experimental data. The partially anomalous K-doublet structure of the S1 state, which deviates from that in a simple torsion-rotation molecule, is fitted using this formalism and is shown to arise from coupling of torsion and rotation motion with the aldehyde-hydrogen inversion.     

Laser-induced fluorescence spectra, structure, and the ring-twisting and ring-bending vibrations of 1,4-benzodioxan in its S0 and S1(pi,pi*) states

The laser-induced fluorescence (LIF) spectra, both the fluorescence excitation spectra (FES) and single vibrational level fluorescence spectra (SVLF) from several different vibronic states, along with the ultraviolet (UV) absorption spectra of 1,4-benzodioxan have been recorded and analyzed. A detailed energy map has been constructed for four low-frequency vibrations and their combinations for both the S(0) and S(1)(pi,pi) electronic states. These are nu(48) (ring-bending), nu(25) (ring-twisting), nu(47) (ring-flapping), and nu(24) (skeletal-twisting). Both the experimental and ab initio calculations show the molecule to be twisted in both the S(0) and S(1)(pi,pi) states with high barriers to planarity. The experimentally determined ring-twisting quantum states, which are confined to the lower regions of the potential energy surface, were used to calculate one-dimensional potential functions in terms of the twisting coordinates, and the extrapolated barriers were estimated to be 5700 and 4200 cm(-1) for the S(0) and S(1) states, respectively. Two-dimensional calculations, which included the interactions with the bending modes, gave values of 3906 and 1744 cm(-1), respectively. The S(0) value compares favorably with the ab initio value of 4095 cm(-1).     

Sub-Doppler rotationally resolved spectroscopy of lower vibronic bands of benzene with Zeeman effects

Sub-Doppler high-resolution excitation spectra and the Zeeman effects of the 6(0)(1), 1(0)(1)6(0)(1), and 1(0)(2)6(0)(1) bands of the S1(1)B2u<--S(0)(1)A1g transition of benzene were measured by crossing laser beam perpendicular to a collimated molecular beam. 1593 rotational lines of the 1(0) (1)6(0) (1) band and 928 lines of the 1(0)(2)6(0)(1) band were assigned, and the molecular constants of the excited states were determined. Energy shifts were observed for the S1(1)B2u(v1=1,v6=1,J,Kl=-11) levels, and those were identified as originating from a perpendicular Coriolis interaction. Many energy shifts were observed for the S1(1)B2u(v1=2,v6=1,J,Kl) levels. The Zeeman splitting of a given J level was observed to increase with K and reach the maximum at K=J, which demonstrates that the magnetic moment lies perpendicular to the molecular plane. The Zeeman splittings of the K=J levels were observed to increase linearly with J. From the analysis, the magnetic moment is shown to be originating mostly from mixing of the S1(1)B2u and S2(1)B1u states by the J-L coupling (electronic Coriolis interaction). The number of perturbations was observed to increase as the excess energy increases, and all the perturbing levels were found to be a singlet state from the Zeeman spectra.     

Mechanism of the photochemical process of singlet oxygen production by phenalenone

Phenalenone (PN) is a very efficient singlet oxygen sensitiser in a wide range of solvents. This work uses ab initio quantum chemical calculations (CASSCF/CASPT2 protocol) to study the mechanism for populating the triplet state of PN responsible for this reaction, the (3)(π-π*) state. To describe in detail this reaction path, the singlet and triplet low-lying excited states of PN have been studied, the critical points of the potential energy surfaces corresponding to these states located and the vertical and adiabatic energies calculated. Our results show that, after the initial population of the S(2) excited state of (π-π*) character, the system undergoes an internal conversion to the (1)(n-π*) state. After populating the dark S(1) state, the system relaxes to the (1)(n-π*) minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the (3)(π-π*) state. Although the population of the minimum of this triplet state is strongly favoured, a conical intersection with the (3)(n-π*) surface opens an internal conversion channel to this state, a path accessible only at high temperatures. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state. Our computational results satisfactorily explain the experimental findings and are in very good agreement with the experimental data available. In the case of the frequency of fluorescence, this is the first time that these data have been theoretically predicted in good agreement with the experimental results.     

Intersystem crossing in the 1nπ* and 1ππ* states

Fast intersystem crossing is observed in the S(1)(1)nπ* state of N-heterocyclic aromatic hydrocarbons and carbonyl compounds. It is attributed to spin-orbit coupling with the (3)ππ* state in the same energy region. The strong singlet-triplet mixing was confirmed by large Zeeman splitting of rotational lines in a high-resolution spectrum. For the S(1)(1)ππ* state of aromatic hydrocarbons, the observed Zeeman splitting was found to be considerably small, and intersystem crossing was considered to be minor. These facts are in accordance with El-Sayed's rule, which states spin-orbit coupling is forbidden between the (1)ππ* and (3)ππ* states. The Zeeman splitting of several derivatives was also observed and the substitution effect on the intersystem crossing rate is discussed.     

Conformational stability from temperature-dependent fourier transform infrared spectra of noble gas solutions, r0 structural parameters, and barriers to internal rotation for ethylamine

Variable temperature (-55 to -145 degrees C) studies of the infrared spectra (3500 to 100 cm(-1)) of ethylamine, CH(3)CH(2)NH(2), dissolved in liquid krypton and/or xenon have been recorded. From these data, the enthalpy differences have been determined to be 54 +/- 4 cm(-1) (0.65 +/- 0.05 kJ/mol), with the trans conformer (methyl group relative to the lone pair of electrons on nitrogen) being the more stable form. It is estimated that there is 61 +/- 1% of the doubly degenerate gauche form present at ambient temperature. The conformational energetics have been calculated with the M?ller-Plesset perturbation method to the second order (MP2(full)) and the fourth order (MP4(SDTQ)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. Basis sets with diffuse functions lead to incorrect prediction of the conformational stability. On the basis of the frequencies of the torsional transitions along with the determined experimental enthalpy difference and gauche dihedral angle, the potential function governing conformational interchange has been obtained, and the determined Fourier cosine coefficients are V(1) = -207 +/- 48, V(2) = 320 +/- 67, V(3) = 1072 +/- 25, V(4) = 55 +/- 11, and V(5) = -96 +/- 28 cm(-1), with a trans-to-gauche barrier of 1286 cm(-1), and a gauche-to-gauche barrier of 715 cm(-1). The 3-fold methyl rotational barriers have been determined to be 1241 +/- 4 and 1281 +/- 10 cm(-1) for the gauche and trans conformers, respectively. By utilizing the previously reported microwave rotational constants combined with the structural parameters predicted at the MP2(full)/6-311+ G(d,p) level, adjusted r(0) structural parameters have been obtained. A complete vibrational assignment is given for the trans conformer, which is supported by normal coordinate calculations utilizing scaled force constants from ab initio B3LYP/6-311++G(3df,3pd) calculations. Proposed assignments are also made for the fundamentals of the gauche conformer. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for similar molecules.