Torsional Barrier Research Articles

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry.

2-Chloro-4-fluorotoluene was investigated using a combination of molecular jet Fourier transform microwave spectroscopy in the frequency range from 5 to 21GHz and quantum chemistry. The molecule experiences an internal rotation of the methyl group, which causes fine splittings of all rotational transitions into doublets with separation on the order of a few tens of kHz. In addition, hyperfine effects originating from the chlorine nuclear quadrupole moment coupling its nuclear spin to the end-over-end rotation of the molecule are observed. The torsional barrier was derived using both the rho and the combined-axis-method, giving a value of 462.5(41) cm-1. Accurate rotational constants and quadrupole coupling constants were determined for the 35Cl and 37Cl isotopologues and compared with Bailey's semi-experimental quantum chemical predictions. The gas phase molecular structure was deduced from the experimental rotational constants supplemented with those calculated by quantum chemistry at various levels of theory. The values of the methyl torsional barrier and chlorine nuclear quadrupole coupling constants were compared with the theoretical predictions and with those of other chlorotoluene derivatives.

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan-2-one.

Methyl n-alkyl ketones form a class of molecules with interesting internal dynamics in the gas-phase. They contain two methyl groups undergoing internal rotations, the acetyl methyl group and the methyl group at the end of the alkyl chain. The torsional barrier of the acetyl methyl group is of special importance, since it allows for the discrimination of the conformational structures. As part of the series, the microwave spectrum of octan-2-one was recorded in the frequency range from 2 to 40 GHz, revealing two conformers, one with C1 and one with Cs symmetry. The barriers to internal rotation of the acetyl methyl group were determined to be 233.340(28) cm-1 and 185.3490(81) cm-1 , respectively, confirming the link between conformations and barrier heights already established for other methyl alkyl ketones. Extensive comparisons to molecules in the literature were carried out, and a small overview of general trends and rules concerning the acetyl methyl torsion is given. For the hexyl methyl group, the barrier height is 973.17(60) cm-1 for the C1 conformer and 979.62(69) cm-1 for the Cs conformer.

Improving Efficiency of Organic Solar Cells by Restricting the Rotation of Side Chain on Small Molecule Acceptor

Herein, three carbazole‐based small molecule acceptors (SMAs), named 4TC‐4F‐C8C8, 4TC‐4F‐C6C8, and 4TC‐4F‐C16, are synthesized to study the influence of side chain conformation on SMAs. The three acceptors exhibit similar optical and electrochemical properties, but different crystallization properties. 4TC‐4F‐C16 shows a high crystallinity due to the small steric hindrance of linear n‐hexadecyl (C16) side chain. The large steric hindrance and free rotation for the 2‐hexydecyl (C6C8) side chain seriously disturb the molecular packing and result in a low crystallinity for 4TC‐4F‐C6C8. Despite the large steric hindrance for the 1‐octylnonyl (C8C8) side chain, 4TC‐4F‐C8C8 shows a moderate crystallinity due to the large torsion barrier restricting the rotation of C8C8 side chain. Attributing to the ideal morphology and better crystalline ordering in blend film, organic solar cell based on poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT):4TC‐4F‐C8C8 displays a power conversion efficiency of 12.06%, higher than PBTIBDTT:4TC‐4F‐C6C8 (2.38%)‐ and PBTIBDTT:4TC‐4F‐C16 (9.53%)‐based devices. The work indicates that controlling the conformation of bulky side chain can tune the molecular packing of SMAs and the morphology of blend film, providing a new insight into the molecular design of SMAs.

Laboratory rotational spectroscopy of methyl n-propyl sulfide: Conformational analysis and methyl internal rotation

The microwave spectrum of methyl n-propyl sulfide, CH3-S-CH2-CH2-CH3, was recorded in the frequency region from 2.0 to 26.5 GHz, revealing three conformers. Quantum chemical calculations were carried out to support the experimental work. Fine splittings arising from the internal rotation of the methyl group attached to the sulfur atom were resolved and analyzed. Torsional barriers of about 600 cm−1 for the two conformers with C1 symmetry and about 700 cm−1 for the Cs conformer could be deduced, showing that the conformations affect the methyl internal rotation. Torsional splittings of the methyl group at the end of the propyl moiety were observed for some transitions, leading to the determination of barrier heights close to 1000 cm−1. The spectrum of the 34S isotopologue of the most stable conformer could be measured in natural abundance. The present laboratory work provides highly accurate spectroscopic parameters, which serve as reliable starting values for extrapolation in higher frequency ranges and for the search of this sulfur-containing molecule in the interstellar medium.

The structure and low-barrier methyl torsion of 3-fluorotoluene.

The rotational spectra of 3-fluorotoluene and its seven 13C isotopic species have been recorded at natural abundance in the frequency range from 4 to 26GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The molecular structure comprising bond lengths and angles as well as parameters describing the methyl torsion were determined with high accuracy. Due to the very low torsional barrier of 17cm-1, the lowest torsional states of the vibrational ground state exhibited large splittings in the spectrum, which were modeled satisfactorily with a modified version of the program XIAM and the program aixPAM, both developed to treat the methyl internal rotation effects. They were also applied to refit the microwave data of 3,4-difluorotoluene to standard deviations close to measurement accuracy.

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene.

Large amplitude motion of methyl groups in isolated molecules is a fundamental phenomenon in molecular physics. The methyl torsional barrier is sensitive to the steric and electronic environment in the surrounding of the methyl group, making the methyl group a detector of the molecular structure. To probe this effect, the microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers of dimethylfluorobenzene, was measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Due to internal rotations of two equivalent methyl groups with relatively low torsional barriers, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922 (21) cm-1 . The results are compared to those obtained from quantum chemical calculations and with other fluorine substituted toluene derivatives of the current literature where the methyl group is adjacent to a fluorine atom.

Use of Extended-Hückel Descriptors for Rapid and Accurate Predictions of Conjugated Torsional Energy Barriers.

Over the past few decades, virtual high-throughput screening (vHTS) and molecular dynamics simulations have become effective and widely used tools in the initial stages of drug discovery efforts. These methods allow a great number of druglike molecules to be screened quickly and inexpensively. Unfortunately, however, the accuracies of both these methods rely on the quality of the underlying molecular mechanics force fields (FFs), which are often poor. This major weakness originates from the reliance of FFs on a finite list of specific parameters, called atom types, which have low transferability between molecules. In particular, the torsional energy barriers of druglike molecules are notoriously difficult to predict. Continuing our endeavor to understand factors affecting the torsional energy barriers of small molecules and quantify them, we showed that descriptors calculated using the extended-Hückel method could be used to rapidly assign accurate torsion parameters for conjugated molecules. This method, called H-TEQ 4.5, was developed using a set of 684 conjugated molecules. It was subsequently validated on a test set of 200 diverse molecules and produced an average root-mean-square error (rmse) of 1.01 kcal·mol-1, with respect to the reference quantum mechanic torsional profiles. For comparison, GAFF2, MMFF94, and MAB produced average rmse's of 3.49, 1.50, and 1.77 kcal·mol-1, respectively. H-TEQ 4.5 is also computationally inexpensive, running just under 0.25 ms for a biphenyl molecule on a home computer, allowing it to be used for vHTS of large libraries of compounds. Overall, H-TEQ 4.5 solved the problems associated with the transferability of torsion parameters for conjugated molecules. This method was incorporated into the Molecular Operating Environment and will be available for a wide variety of applications.

Difficulties of Popular Density Functionals to Describe the Conformational Isomerism in Iodoacetic Acid.

Matrix isolation studies in solid argon and neon at 4.2 K reveal that iodoacetic acid initially only exists as its ground state (c,x) conformer with an almost perpendicular I-C-C═O dihedral angle, but UV irradiation in the 240-255 nm range leads to population of the 0.8 kcal mol-1 less stable (c,c) isomer. The latter structure exhibits a close 3.23 Å contact of the iodine and carbonyl oxygen atoms decidedly below the sum of their van der Waals radii (3.50 Å). Increasing the matrix temperature by only a few Kelvin triggers the thermal back reaction of (c,c) to (c,x) and leads to an estimated upper limit of 0.38 kcal mol-1 for the associated torsional barrier. While wave function methods including completely uncorrelated Hartree-Fock theory have no problem to identify (c,c) as a proper minimum, many popular density functionals fail to describe the C-C torsional potential in cis-iodoacetic acid qualitatively correct. We assessed the performance of 12 density functionals of different levels of sophistication, namely, the BLYP, PBE, TPSS, B3LYP, BHandHLYP, PBE0, M06-2X, CAM-B3LYP, ωB97X-D3, B2-PLYP, B2GP-PLYP, and DSD-PBEP86 methods, against accurate extrapolated CCSD(T)/CBS(T-Q)//MP2/def2-TZVPP energies and found that almost all of them yield acceptable relative energies. Still, even some of the best performers fail to find a reasonably deep minimum in the region of the (c,c) conformer, and addition of the empirical D3-dispersion correction does not remedy the qualitative shortcoming. Instead, inclusion of a sufficient amount of (long-range) exact exchange and likely a proper treatment of medium-range correlation effects all along the torsional coordinate play an important role in the proper description of the sub-van der Waals iodine-oxygen contact. More modern, recommended functionals do not suffer from the described shortcoming.

Photoisomerization dynamics of trans-trans, cis-trans, and cis-cis diphenylbutadiene from broadband transient absorption spectroscopy and calculations.

The photoisomerization path and dynamics of trans-trans (ttD), cis-trans (ctD), and cis-cis (ccD) 1,4-diphenyl-1,3-butadiene (DPB) in solution are studied with broadband transient absorption (TA) spectroscopy and quantum chemical calculations. For ttD in n-hexane, 2-photon-excited TA spectra indicate that the 2Ag state is located above 1-photon allowed 1Bu (S1) by ∼1000 cm-1. Following S0 → S1 optical excitation, the isomerization occurs via torsion about a butadiene double bond to perpendicular molecular configuration P. The P-state is detected in ccD with an excited-state absorption band at 390 nm. This P-band develops during S1 → P half-torsion with time of 0.15 ps, followed further by P → S0 half-torsion and simultaneous decay with 1.6 ps in acetonitrile and 5 ps in n-hexane. In addition, two oscillation cycles between P and S1 population are observed before equilibration in n-hexane. For ctD, an indication of rising and decaying P is found in acetonitrile. The vast majority of ctD species photoisomerizes to ttD and not to ccD, in agreement a with calculated low torsional barrier about the cis double bond and high barrier about the trans double bond. Photoisomerization yield Y and time τi depend drastically on the solvent polarity. Thus, in n-hexane, the isomerization ttD → ctD has yield Ytt,ct = 0.1 and time τi = 829 ps, while in acetonitrile, Ytt,ct = 0.4 and τi = 27 ps. The 30-fold acceleration of the isomerization in acetonitrile clearly reflects a highly polar character of P, consistent with a dipole moment μP > 9.6 D. The results for DPB are discussed in comparison to stilbene.

Discovery of full color emissive thiazole fluorophores in solution and solid states: The core is central and regulating torsional barrier does the trick!

The rare class of full color tunable molecules from 5-(hetero-2-yl)-1,3-thiazole core with solid and solution state emissions were designed. Stable conformers were predicted computationally and C4 substituent on the 1,3-thiazole ring emerged crucial as multifunctional stacking modulator in the multidirectional charge transfer donor-acceptor systems. Seven novel molecules were synthesized, and their emissions with very good quantum yield both in solution and solid states were reported along with very rare full color dual emission (solution:469–672 nm, solid:466–614 nm). These full color dual state emissive (FCDSE) molecules also displayed solvatochromism and large Stokes shift (solution: 78–210 nm, solid: 90–106 nm). Further, absorption and emission wavelengths were calculated using TDDFT-PBE0/6-31G(d,p) by accounting the solvent effects using Poisson Boltzmann solvation model and the experimental results were well reproduced. The accurate prediction of absorption-emission trend in this homologous series is expected to guide the design of custom-emissive fluorophores and the facile synthetic route can provide diverse compounds for desired technological applications.

Conformer Specific Excited-State Structure of 3-Methylthioanisole.

Trans and cis conformers of 3-methylthioanisole have been spectroscopically investigated to reveal the conformer specific structural changes upon the S1(ππ*)-S0 excitation. The conformational cooling during the supersonic expansion is found to be quite efficient in the Ar carrier gas giving the trans conformational isomer exclusively in the molecular beam, whereas both trans and cis conformers are populated in the jet when the sample is carried in Ne. Using the Stark deflector, trans and cis conformers are unambiguously identified, showing the distinct Stark deflection profiles according to their sufficiently different dipole moments of 1.013 or 1.670 D, respectively. For the trans conformer, the methyl moiety on the meta-position adopting the eclipsed geometry in S0 transforms into the staggered geometry in S1 to activate a series of the CH3 torsional mode. A Hamiltonian with the one-dimensional sinusoidal torsional potential is solved using the free-rotor basis set to explain the experiment, giving the 3-fold torsional barrier of 34 and 304 cm-1 for S0 and S1, respectively. For the cis conformer, on the other hand, the CH3 torsion is little activated in the S1-S0 transition as both S0 and S1 adopt the staggered geometry at the minimum energy points. The doublet of each band of the cis conformer is ascribed to tunneling split due to the very low CH3 torsional barrier of 27 cm-1 in S0. It is found that the cis conformer undergoes a planar to pseudoplanar structural change upon the S1-S0 transition. Theoretical calculation based on the double-well model potential curve could explain the experiment quite well, suggesting that the SCH3 moiety of the cis conformer in S1 becomes out-of-plane with respect to the plane of the phenyl moiety. This implies that excited-state predissociation dynamics of trans and cis conformers of the title molecule might be different.

Exploiting a Mechanical Perturbation of a Titin Domain to Identify How Force Field Parameterization Affects Protein Refolding Pathways.

Molecular mechanics force fields have been shown to differ in their predictions of biomolecular processes such as protein folding. To test how force field differences affect predicted polypeptide behavior, we created a mechanically perturbed model of the β-stranded I91 titin domain based on atomic force spectroscopy data and examined its refolding behavior using six different force fields. We found that different force fields varied significantly in their ability to refold the mechanically perturbed I91 domain. Examination of the perturbed I91 unfolded state revealed that all five Amber force fields oversample a specific region of the Ramachandran plot, thereby creating unfolded state intermediates which are not predicted by the Charmm 22* force field. Simulations of perturbed I91 refolding with Amber FB15 revealed that Amber FB15 destabilizes stable portions of I91, thereby contradicting experimental stability analyses. Finally, inspection of the perturbed I91 unfolded state along with equilibration simulations of the Ac-(AAQAA)3-NH2 peptide suggest that high dihedral torsional barriers cause the Amber ff14SB force field to predict higher helical lifetimes relative to other force fields. These results suggest that using mechanically perturbed models can provide a controlled method to gain insights into how force fields affect polypeptide behavior.

The microwave spectrum of 2-methylthiazole: 14N nuclear quadrupole coupling and methyl internal rotation.

The rotational spectrum of 2-methylthiazole was recorded using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range of 2-40 GHz. Due to the internal rotation of the methyl group, all rotational transitions were split into A and E symmetry species lines, which were analyzed using the programs XIAM and BELGI-Cs-hyperfine, yielding a methyl torsional barrier of 34.796 75(18) cm-1. This value was compared with that found in other monomethyl substituted aromatic five-membered rings. The 14N quadrupole coupling constants were accurately determined to be χaa = 0.5166(20) MHz, χbb - χcc = -5.2968(50) MHz, and χab = -2.297(10) MHz by fitting 531 hyperfine components. The experimental results were supplemented by quantum chemical calculations.

Next level achievement of the XIAM code in modeling the microwave spectrum of m-methylanisole

The XIAM code is one of the most frequently used programs to treat the microwave spectra of molecules with up to three methyl internal rotors. XIAM is user-friendly and fast, but often shows difficulties in dealing with low torsional barriers. An example is the case of m-methylanisole where the methyl group attached at the meta-position of the ring undergoes internal rotation with a barrier height of about 56 cm−1 for the cis conformer and 37 cm−1 for the trans conformer. The standard deviation obtained with XIAM is much larger than the measurement accuracy for both conformers. Recently, the code has been modified, and three higher order effective parameters connected to the potential term cos(3α) were implemented, which reduced the standard deviations of the fits to almost measurement accuracy.

Vibrational Dynamics of Crystalline 4-Phenylbenzaldehyde from INS Spectra and Periodic DFT Calculations.

The present work emphasizes the value of periodic density functional theory (DFT) calculations in the assessment of the vibrational spectra of molecular crystals. Periodic calculations provide a nearly one-to-one match between the calculated and observed bands in the inelastic neutron scattering (INS) spectrum of crystalline 4-phenylbenzaldehyde, thus validating their assignment and correcting previous reports based on single molecule calculations. The calculations allow the unambiguous assignment of the phenyl torsional mode at ca. 118–128 cm−1, from which a phenyl torsional barrier of ca. 4000 cm−1 is derived, and the identification of the collective mode involving the antitranslational motion of CH···O bonded pairs, a hallmark vibrational mode of systems where C-H···O contacts are an important feature.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH+ ), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH+ and m-OH-BZH+ only cis conformers are present in the ion population analyzed. For o-OH-BZH+ , IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis-trans isomerization calculated for each OH-BZH+ isomer is a measure of the degree of π-electron delocalization. Furthermore, IRMPD spectra of p-OH-BZH+ , m-OH-BZH+ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appear to be a measure of π-electron delocalization in the ions.

Experimental and theoretical studies of the doubly substituted methyl-ethyl Criegee intermediate: Infrared action spectroscopy and unimolecular decay to OH radical products.

The infrared (IR) action spectrum of the doubly substituted methyl-ethyl Criegee intermediate (MECI) is observed in the CH stretch overtone region with detection of OH products. The MECI exhibits four conformers, all of which undergo unimolecular decay via a 1,4 H-atom transfer mechanism, followed by the rapid release of OH products. Conformers with different orientations of the carbonyl oxide group with respect to the methyl and ethyl substituents (i.e., anti and syn) decay via distinct transition state barriers (16.1 kcal mol-1 and 15.4 kcal mol-1, respectively). The observed IR action spectrum is in good agreement with the predicted anharmonic IR absorption spectrum, but exhibits significant congestion, which is attributed to couplings between spectroscopic bright states and nearby dark states. Energy-dependent OH appearance rates are measured upon IR excitation of the strongest features in the IR action spectrum and are found to be on the order of 106-107 s-1. The experimental rates are in good agreement with computed Rice-Ramsperger-Kassel-Marcus rates for the unimolecular decay of MECI at these energies, which incorporate quantum mechanical tunneling and sophisticated hindered rotor treatments, as well as high-level theoretical calculations of the TS barrier heights, rovibrational properties, and torsional barriers associated with the MECI conformers. Master equation modeling is used to predict thermal rates for the unimolecular decay of anti- and syn-MECI of 473 s-1 and 660 s-1, respectively. Comparison with other previously studied Criegee intermediate systems provides insights into substituent effects on unimolecular decay under both energy-dependent and thermal conditions.

Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy.

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm-1 and 584-588 cm-1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.

Transient Rotamerism and Photoisomerization Dynamics of trans- and cis-Naphthylstilbene.

The rotamerism and photoisomerization of trans- and cis-1,2-di-(2-naphthyl)ethylene (tN and cN) are studied with stationary and transient absorption spectroscopies assisted by quantum chemical calculations. Absorption and emission spectra of rotamers (rotational isomers) tN-S (C2h-symmetric), tN-A (C1), and tN-S' (C2) are derived with a 53:47 ratio of tN-S to tN-A. Upon photoexcitation, the equilibration of the rotamers in S0 (rotamerization) is observed in the bleach region with characteristic time τrotamer ≈ 0.5 ns. With excitation at 364 nm, the S0 equilibrium shifts because, mainly, tN-A is bleached and the rotamerization becomes traceable, whereas with excitation at 345 nm, the equilibrium is preserved and the bleach spectrum remains unchanged. It is just long-lived (∼2 ns) S1 that allows for monitoring the rotamer dynamics in S0. Replacement of the stilbene phenyl rings with larger naphthyls increases the S1 → P torsional barrier E1act toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with τR ≈ 3.7 ns becomes the main deactivation channel, and accordingly, the measured decays show nearly no dependence on the solvent viscosity. The cis-to-trans photoisomerization occurs via two paths: adiabatic cS1 → P → tS1 (20%) and more common nonadiabatic cS1 → P → S0 (80%). The barrier cS1 → P in the cis-isomer is reduced in polar solvents because of a zwitterionic character of P. The P-state is directly detected with the cN isomer in acetonitrile by an excited-state absorption band at 400 nm developing with 0.7 ps and decaying with 1.6 ps. Two dihydrophenanthrene (DHP)-like products result from photoexcited cN. The metastable DHP-A builds up transiently from cN-A, and its spectrum at about 550 nm matches the published DHP absorption. The stable DHP-S' accumulates under stationary illumination and is formed either from excited cN-S' or metastable DHP-A.

Ethylene Intersystem Crossing Caught in the Act by Photofragment Sulfur Atoms

Ethylene, C2H4, the simplest π-bonded molecule, is of enormous fundamental and commercial importance. Its lowest triplet state, in which the CH2 moieties occupy perpendicular planes, is well known from theory, but there has been no definitive experimental observation of this species. Here, velocity map imaging of the sulfur atoms in ethylene sulfide (c-C2H4S) photodissociation at 217 nm is used to reveal the internal state distribution of co-product ethylene. While both S (1D) and S (3P) translational energy distributions display three distinct regions that find their origins in singlet and triplet excited states of c-C2H4S, respectively, the S (3P) distribution is dominated by a fourth, low-recoil region. In this region, the distribution is fully isotropic at a recoil of 9 ± 1 kcal/mol, corresponding to the opening of the triplet ethylene channel. Multireference calculations suggest that this photodissociation pathway is mediated by a hot, transient biradical CH2CH2S that strongly favors CH2-hindered rotations in the predissociated complex. This photochemical ring-opening mechanism is invoked to account for the vibrational features observed in this low-recoil region, which are attributed to triplet ethylene relaxing to the torsional saddle point on the ground-state singlet surface. This study thereby gives for the first time the experimental confirmation of an adiabatic singlet-triplet splitting of 66 ± 1 kcal/mol and a torsional barrier height of 64 ± 1 kcal/mol in ethylene.