Ground State Trans Research Articles

Color detection using chromophore-nanotube hybrid devices.

We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism.

Tracking Ultrafast Excited-State Bond-Twisting Motion in Solution Close to the Franck−Condon Point

Applying optimal control to photoinduced trans-cis isomerization in condensed phase, the dynamics of bond-twisting motion of 1,1'-diethyl-4,4'-cyanine in methanol and propanol is revealed. The shape of the optimized pulse resulting from minimization of the photoisomer formation can be directly related to the initial excited-state dynamics in close proximity to the Franck-Condon point. The solvent viscosity-dependent ultrafast wavepacket motion is reflected in the prominent down-chirp of the optimized pulses and reveals a detailed picture of the control mechanism: The reduction of the isomer production is achieved by most efficient dumping of excited population back to the trans ground state. In the higher-viscosity solvent, propanol, wavelength-dependent oscillatory features are superimposed to the overall chirp structure pointing to the importance of excited-state vibrational coherences for the dumping process.

Competing ultrafast cis-trans isomerization and ring closure of cyclohepta-1,3-diene and cyclo-octa-1,3-diene

Photochemically, s-cis-dienes can undergo cis-trans isomerization or electrocyclic ring closure to cyclobutenes. Modern quantum chemistry predicts that both reactions can originate from the same conical intersection and explains thereby why photochemical ring opening of cyclobutenes to dienes is often not stereospecific, although ring closure of dienes is. Evidence is found that the reaction path already branches earlier, investigating two cyclic dienes which were excited by a femtosecond UV pulse in the gas phase and probed by photoionization at 810 nm. The multistep path is assigned to the excited-state surfaces and ring closure of the ground-state trans isomers is also detected.

The Profound Influence of a Single Metal‐Carbon Bond on the Reactivity of Bio‐Relevant Cobalt(III) Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Ar

Acetic acid monomer has two stable geometries, the cis and trans conformers. The high-energy cis conformer has been recently detected experimentally for the first time [Maçôas et al. J. Am. Chem. Soc. 2003, 125, 16188]. The cis conformer can be produced in low-temperature rare-gas matrixes upon vibrational excitation of the ground-state trans conformer. Fast tunneling from cis- to trans-acetic acid takes place even at the lowest working temperatures (8 K), limiting the time available to study the high-energy form. Deuteration of the hydroxyl group reduces the tunneling rate by approximately 4 orders of magnitude, increasing accordingly the lifetime of the unstable conformer and its available concentration. In this work, we present a detailed analysis of the vibrational spectra of the cis form of four acetic acid isotopologues (CH3COOH, CH3COOD, CD3COOH and CD3COOD). Photolysis (193 nm) of the trans and cis forms of the perdeuterated compound was performed to evaluate the possible conformational dependence of photodissociation of acetic acid. However, no evidence of conformer specific photodissociation was found. The UV photolysis of the matrix-isolated acetic acid reveals very different products from the gas phase. Methanol complexed with carbon monoxide is the major product of photolysis of acetic acid isolated in Ar matrixes whereas it has never been observed as a photolysis product in the gas phase.

Photoactivation of the photoactive yellow protein: why photon absorption triggers a trans-to-cis Isomerization of the chromophore in the protein.

Atomistic QM/MM simulations have been carried out on the complete photocycle of Photoactive Yellow Protein, a bacterial photoreceptor, in which blue light triggers isomerization of a covalently bound chromophore. The "chemical role" of the protein cavity in the control of the photoisomerization step has been elucidated. Isomerization is facilitated due to preferential electrostatic stabilization of the chromophore's excited state by the guanidium group of Arg52, located just above the negatively charged chromophore ring. In vacuo isomerization does not occur. Isomerization of the double bond is enhanced relative to isomerization of a single bond due to the steric interactions between the phenyl ring of the chromophore and the side chains of Arg52 and Phe62. In the isomerized configuration (ground-state cis), a proton transfer from Glu46 to the chromophore is far more probable than in the initial configuration (ground-state trans). It is this proton transfer that initiates the conformational changes within the protein, which are believed to lead to signaling.

Probing the ruthenium-cumulene bonding interaction: synthesis and spectroscopic studies of vinylidene- and allenylidene-ruthenium complexes supported by tetradentate macrocyclic tertiary amine and comparisons with diphosphine analogues of ruthenium and osmium.

The synthesis and spectroscopic properties of trans-[Cl(16-TMC)Ru[double bond]C[double bond]CHR]PF(6) (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, R = C(6)H(4)X-4, X = H (1), Cl (2), Me (3), OMe (4); R = CHPh(2) (5)), trans-[Cl(16-TMC)Ru[double bond]C[double bond]C[double bond]C(C(6)H(4)X-4)(2)]PF(6) (X = H (6), Cl (7), Me (8), OMe (9)), and trans-[Cl(dppm)(2)M[double bond]C[double bond]C[double bond]C(C(6)H(4)X-4)(2)]PF(6) (M = Ru, X = H (10), Cl (11), Me (12); M = Os, X = H (13), Cl (14), Me (15)) are described. The crystal structures of 1, 5, 6, and 8 show that the Ru-C(alpha) and C(alpha)-C(beta) distances of the allenylidene complexes fall between those of the vinylidene and acetylide relatives. Two reversible redox couples are observed by cyclic voltammetry for 6-9, with E(1/2) values ranging from -1.19 to -1.42 and 0.49 to 0.70 V vs Cp(2)Fe(+/0), and they are both 0.2-0.3 and 0.1-0.2 V more reducing than those for 10-12 and 13-15, respectively. The UV-vis spectra of the vinylidene complexes 1-4 are dominated by intense high-energy bands at lambda(max) < or = 310 nm (epsilon(max) > or = 10(4) dm(3) mol(-1) cm(-1)), while weak absorptions at lambda(max) > or = 400 nm (epsilon(max) < or = 10(2) dm(3) mol(-1) cm(-1)) are tentatively assigned to d-d transitions. The resonance Raman spectrum of 5 contains a nominal nu(C[double bond]C) stretch mode of the vinylidene ligand at 1629 cm(-1). The electronic absorption spectra of the allenylidene complexes 6-9 exhibit an intense absorption at lambda(max) = 479-513 nm (epsilon(max) = (2-3) x 10(4) dm(3) mol(-1) cm(-1)). Similar electronic absorption bands have been found for 10-12, but the lowest energy dipole-allowed transition is blue-shifted by 1530-1830 cm(-1) for the Os analogues 13-15. Ab initio calculations have been performed on the ground state of trans-[Cl(NH(3))(4)Ru[double bond]C[double bond]C[double bond]CPh(2)](+) at the MP2 level, and imply that the HOMO is not localized purely on the metal center or allenylidene ligand. The absorption band of 6 at lambda(max) = 479 nm has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal nu(C[double bond]C[double bond]C) stretch mode accounts for ca. 50% of the total vibrational reorganization energy, indicating that this absorption band is strongly coupled to the allenylidene moiety. The excited-state reorganization of the allenylidene ligand is accompanied by rearrangement of the Ru[double bond]C and Ru[bond]N (of 16-TMC) fragments, which supports the existence of bonding interaction between the metal and C[double bond]C[double bond]C unit in the electronic excited state.

The profound influence of a single metal-carbon bond on the reactivity of bio-relevant cobalt(III) complexes.

This Perspective reports the development of mechanistic insight over the past 6 years, on the substitution behaviour of cobalamins that contain a single Co-C bond. The effect of the alkyl group, located in the trans position, on the thermodynamic, kinetic and ground state trans effect, was studied in detail. The substitution reactions of different alkylcobalamins with CN- were investigated, the apparent mechanistic discrepancy reported for the co-enzyme B12 was resolved and a logical explanation could be offered. In addition, a complete picture of the effect of pressure on the UV-Vis spectra of different base-on and base-off cobalamins is presented, which clearly shows the role of the alkyl group in controlling the equilibrium between five- and six-coordinate species, and the possible participation of such species in the studied ligand substitution reactions. The kinetics of the base-on/base-off equilibration was studied for the first time using a pH-jump technique. All in all the novel mechanistic information adds to the understanding of the profound effect that a single metal-carbon bond can have on the reactivity of such Co(III) complexes.

Photoexcited triplets of nitrodiphenylpolyenes hosted in channel cavities. A time resolved EPR study

The photoexcited triplet states of the three nitro substituted diphenylpolyenes 4-nitrostilbene, 4-nitrodiphenylbutadiene and 4,4′-dinitrodiphenylbutadiene included in nanostructured channel-like cavities of a PHTP-based inclusion compound have been investigated by time-resolved electron paramagnetic resonance. The inclusion compounds were synthesized by using as host dihydrotriphenylene partially hydrogenated. The same dyes have been studied also in disordered solid matrices. The triplet states produced in the inclusion compounds originate only from the ground state trans conformer, since the constraint of the host channel prevents the trans–cis photoisomerization. The simulation of all spectra in disordered matrices shows the presence of the same triplet from the trans isomer as in inclusion compounds, but also of another state with the same ZFS parameters and different spin polarization. This derives from the cis isomer through a photophysical path to the cis triplet relaxing therefore conformationally to the trans triplet. For the diphenylpolyene derivatives the effect of irradiation at two wavelengths has been studied, giving further support to this interpretation of the spectra. Comparison of the experimental ZFS parameters with the calculated ones and with those of the corresponding unsubstituted compounds shows that the nitro groups are tilted out of the π-system plane. The tilting angle is different in the inclusion compounds with respect to the disordered matrices as a consequence of the head-to-tail interactions between nitro groups in neighbour molecules packed in the channels.

Bridge-splitting kinetics, equilibria and structures of trans-biscyclooctene complexes of platinum(ii)Electronic supplementary information (ESI) available: observed pseudo-first order rate constants for bridge splitting reactions, absorbance versus added ligand concentrations for equlibrium constant determinations and complete crystallographic details in CIF format. See http://www.rsc.org/suppdata/dt/b3/b302482m/

Cyclooctene (C8H14, cot) complexes of platinum(II) have been synthesised and characterised by means of multinuclear NMR, UV-Vis spectroscopy and X-ray crystallography. The bridge-splitting equilibrium constants of the dinuclear, chloride-bridged trans-[PtCl2(cot)]2, 1, in dichloromethane solvent at 298 K with MeOH, MeCN and cot, yielding trans-[PtCl2(cot)(MeOH)], 2, trans-[PtCl2(cot)(MeCN)], 3, and trans-[PtCl2(cot)2], 4, were determined by UV-Vis measurements as K12 = 0.0169 ± 0.0015, K13 = 9.7 ± 0.9 and K14 = 2.05 ± 0.06 mol−1 dm3, respectively. Substitution of one cyclooctene from 4 by MeOH gives K42 = 0.110 ± 0.009 and by MeCN K43 = 2.25 ± 0.03. The bis-cyclooctene complexes 1 and 4 react quantitatively with chloride to give [PtCl3(cot)]−, 5. The kinetics for bridge-splitting of 1 with MeOH, MeCN and cot was studied by conventional and cryo-stopped-flow spectroscopy. Second-order rate constants at 298 K are 0.128 ± 0.003, 4.93 ± 0.02 and 0.0637 ± 0.0009 mol−1 dm3 s−1, respectively. The corresponding activation parameters are ΔH≠ = 43.7 ± 1.8, 42.0 ± 0.8 and 39.6 ± 0.9 kJ mol−1 and ΔS≠ = −115 ± 6, −91 ± 3 and −135 ± 3 J K−1 mol−1, indicating associative activation with a relatively large -TΔS≠ contribution to ΔG≠. Crystal and molecular structures of 1, 4, and the tetraphenylphosphonium salt of 5 have been determined, indicating a moderate ground-state trans influence of cyclooctene. The molecular structure of 4 with two CC double bonds coordinated trans to each other and perpendicular to the platinum coordination plane, features a significant Pt–C bond lengthening compared to cyclooctene complexes with chloride or nitrogen trans to cyclooctene, leading to high reactivity and thermodynamic instability. Equilibrium data confirm that the thermodynamic stability of a cyclooctene coordinated trans to another cyclooctene is much lower than trans to a ligand with less π-back-bonding capacity.

Infrared spectra of (NO)2+, (NO)2−, and (NO)3− trapped in solid neon

New studies of the infrared spectra of the products which result on codeposition at approximately 5 K of a Ne:NO sample with Ne atoms that have been excited in a microwave discharge have led to new and revised assignments for several ionic species. The appearance of the ν1 absorption of ONNO+ for several new species with asymmetric isotopic substitution, but for no symmetrically substituted species, confirms the trans ground-state configuration for ONNO+. The behavior of a neon-matrix product absorption at 1227.5 cm−1 parallels that of an argon-matrix absorption at 1221.0 cm−1 which has recently been assigned to trans-ONNO−. The identity of the carrier of a product absorption at 1424.1 cm−1, contributed by a vibration of two symmetrically equivalent NO groups, has not been definitively established. This absorption exhibits complex photodestruction behavior. Three absorptions are assigned to cis,cis-(NO)3−, which has C2v symmetry, with the aid of density functional calculations of the isotopic substitution pattern for the vibrational fundamentals of this species. Similar calculations of the isotopic substitution patterns for other structures result in poor agreement with the experiments. Photodestruction of cis,cis-(NO)3− trapped in solid neon yields the N2O⋯NO2− complex.

Photochemistry of 1-cyclohexenyl phenyl ketone in enol ether solvents. Trapping of the primary photoproduct by formation of [4+2] adducts in high yields

Photolysis (>300 nm) of 1-cyclohexenyl phenyl ketone ( 1) in ethyl vinyl ether (EVE) or in the methyl homologues of EVE, ethyl E-1-propenyl ether and ethyl Z-1-propenyl ether, in each case gives exactly two stereoisomers of the [4 + 2] head-to-head adducts of the enol ether double bond to the olefin-carbonyl moiety of 1. The two stereoisomers obtained from the E-propenyl ether are different from the two stereoisomers obtained from the Z-propenyl ether; all four of them arise from suprafacial addition to the enol ether double bond. As shown in some detail in the case of EVE, besides the two [4 + 2] adducts smaller amounts of other products are formed, some of which have been known before from the enol ether-free system, which stem from the oxyallyl 3 formally derived from 1 by cyclisation. Absolute and relative quantum yields of formation of the diverse products in dependence on EVE concentration allow the conclusion that the photo-intermediate A that gives the [4 + 2] adducts with EVE, does so in competition with both reversion of A to ground state 1 and with ring closure of A to 3. It is concluded that A is the highly strained ground-state trans double-bond isomer of 1, viz. 2, formed from 1 with a quantum yield of ca. 0.36 in toluene–EVE mixtures. Even in neat EVE, reversion to 1 accounts for about half of the 2. The cyclisation of 2 to 3 is about three times faster in the polar acetonitrile than in the nonpolar toluene, relative to reversion to 1.

The transition state in the isomerization of rhodopsin

We present an ab-initio molecular dynamics study of a model of the chromophore of rhodopsin. The 11- cis to all- trans ground-state isomerization of a retinylidene–ethylimine·HCl has been induced by applying an external classical force field to the C10–C11–C12–C13 dihedral angle, in addition to the ab-initio forces. By using this hybrid approach, we find that the transition state is characterized by a well-defined bond pattern along the chromophore and by a positive charge displacement. The relaxation of the chromophore is associated with propagation of a conjugation defect that transports the excess positive charge along the backbone.

Trans cis isomerization in long linear polyenes as beta-carotene models: a comparative CAS-PT2 and DFT study

This computational density functional theory (DFT) study provides information about the ground state trans →cis isomerization process for a set of all-trans linear conjugated polyenes including the undecaene term which can be considered a model of β-carotene. To obtain information about the accuracy of the isomerization barriers computed with the DFT approach we have compared, for a series of shorter all-trans polyenes, the DFT results with those obtained using the CAS-PT2 approach. This comparison has shown that the DFT barriers agree with the CAS-PT2 ones within 1 kcalmo-1. We found for the undecaene term that the singlet ground state central double bond isomerization process occurs through a flat twisted transition state with small rotational barrier (22·9 kcal mol-1) and no special stabilizing effects. We have also investigated, at the DFT level, the trans→cis isomerization process in the triplet state of undecaene and the DFT results show again two planar low-lying minima (a more stable all-trans minimum and a central bond-cis minimum) connected by a twisted transition state (having the same geometry and energy of the twisted singlet ground state transition structure) with an unfavourable intersystem crossing between the two states. These results suggest that the deactivating ability of β-carotene is associated with the attack of the reactive species upon the two singlet closed shell minima.

Photochemistry of 7,7-disubstituted 3,4,4 a,5,6,7-hexahydro-4 a-methyl-1(2 H)-naphthalenones

The photochemistry of (±)-3,4,4 a,5,6,7-hexahydro-4 a-methyl-7,7-diphenyl-1(2 H)-naphthalenone ( 1) and (±)-3,4,4 a,5,6,7-hexahydro-4 a-7,7-trimethyl-1(2 H)-napthalenone ( 2) is presented. Both molecules incorporate a rigid s- cis enone extended over two rings. In benzene, substrates 1 and 2 were photoinert. In alcohol solvents, hydrogen-bonded enols resulting from solvent addition to the enone doubel bond were isolated as the only photoproducts. The rearrangements anticipated for γ,γ-disubstituted enones, aryl migration for 1 and Type A rearrangement for 2, were not observed. Spectroscopic and X-ray structure elucidation showed that solvent addition is highly regio- and stereospecific. Ketonization of the less hindered enols derived from 2 was observed upon heating or exposure to silica gel; enols produced from 1 were stable. Photolysis of 1 and 2 in aqueous dioxane led exclusively to β-hydroxy ketones. The mechanism leading to product formation was rationalized in terms of a stepwise addition of solvent to a triplet-derived ground state trans enone. A mechanism involving direct addition of solvent to a polarized excited state was also considered.

Synthesis, Structure, and Reactivity of Arylchlorobis(dialkyl sulfide)platinum(II) Complexes.

Complexes trans-[PtRCl(SR'(2))(2)], where R = Ph, mesityl, and p-anisyl and R' = Me or Et, have been synthesized and their crystal and molecular structures determined. Crystals of trans-[PtPhCl(SEt(2))(2)] (2) are triclinic (P&onemacr;) with a = 10.112(6) Å, b = 13.158(2) Å, c = 14.714(5) Å, alpha = 102.48(2) degrees, beta = 94.394(4) degrees, gamma = 90.22(3) degrees, and Z = 4. Crystals of trans-[Pt(mesityl)Cl(SMe(2))(2)] (4) are monoclinic (P2(1)/c) with a = 13.158(2) Å, b = 9.170(1) Å, c = 16.013(3) Å, beta = 120.93(2) degrees, and Z = 4, and crystals of [Pt(p-anisyl)Cl(SMe(2))(2)] (5) are monoclinic (P2(1)/n) with a = 9.879(4) Å, b = 8.128(2) Å, c = 19.460(5) Å, beta = 96.56(3) degrees, and Z = 4. All complexes are square-planar, featuring Pt-Cl distances between 2.40 and 2.42 Å, indicating a large ground-state trans influence of the aryl group. The coordination geometry is maintained in methanol and chloroform solution as shown by (1)H-NMR spectra. The kinetics of substitution of the labile chloride trans to aryl by various nucleophiles has been studied in methanol by variable-temperature and -pressure stopped-flow spectrophotometry. A two-term rate law with a well-developed solvolytic pathway is followed. Negative entropies and volumes of activation indicate an associative mode of activation in all cases, independent of steric blocking of the axial sites and a large Pt-Cl ground-state bond-weakening. Comparison of the reaction rates of the present series of complexes with their bis(phosphine) analogues and with related cyclometalated compounds shows that the triethylphosphine complexes are 2-3 orders of magnitude less reactive than the thioether complexes, which in turn are a factor 10-20 less reactive than the cyclometalated ones. This reactivity increase can be rationalized mainly in terms of a decrease in steric hindrance in the series. There seems to be no inherent differences with regard to trans labilizing ability of the aryl ligands in the various types of complexes, including the cyclometalated ones.

UV Resonance Raman Ground and Excited State Studies of Amide and Peptide Isomerization Dynamics

We report the first measurements of the activation barrier for ground state trans−cis isomerization of secondary amides. We measured activation barriers of Ea = 13.8 ± 0.8 kcal/mol for aqueous solu...

Trans Effect and trans influence of triphenyl-stibine and -phosphine in platinum(II) complexes. A comparative mechanistic and structural study ‡

The kinetics and mechanism of the reactions between trans-[PtI3(PPh3)]– and trans-[PtI3(SbPh3)]– with pyridine, 2- and 4-methylpyridine in acetonitrile solvent have been studied by stopped-flow spectrophotometry. The crystal and molecular structures of the tetrabutylammonium salts of the two anions have been determined. Substitution of iodide trans to stibine is reversible and takes place via parallel direct and solvolytic pathways; substitution in the phosphine complex occurs with negligible back reaction. The kinetic data indicate that triphenyl-stibine and -phosphine should be placed in a series of decreasing trans effect, C2H4 > SbPh3 > CO > P(OMe)3 > PPh3 > AsEt3, i.e. SbPh3 has a much larger trans effect than that of PPh3; stibine complexes in the present study react ca. 16 times faster than their phosphine analogues. The activation parameters are typical of associatively activated processes, and in the case of the stibine complex they indicate that very little bond breaking has occurred in the transition state. In the ground states there is a clear-cut difference in the Pt–I distances trans to the pnictogen, 2.637(2) A in the stibine complex and 2.662(3) A in the phosphine complex, indicating that SbPh3 has a smaller ground-state trans influence and hence is a weaker σ donor than PPh3. Since the kinetic trans effect is a combination of ground-state labilisation and transition-state stabilisation, it is concluded that the large trans effect of stibine is due to a better π acceptance. Based on a comparison of Sb–C distances and C–Sb–C angles in free and co-ordinated stibine, this is proposed to be due to a higher d character of the π* orbitals on stibine as compared to phosphine, leading to a better overlap between antimony π* and platinum 5d π orbitals.

Evidence for the stabilization of rectangular O+4 in solid neon

The reassignment of the 1320 and 2949 cm−1 absorptions of O+4, previously attributed to the trans isomer, is considered. A satisfactory least-squares force constant fit of the isotopic data for these two absorptions, assuming a rectangular structure, has been obtained. In accord with the results of ab initio calculations by Lindh and Barnes, it is concluded that, as previously proposed, the 1164 and 2808 cm−1 absorptions should be assigned to the trans ground state of O4+O, but that the rectangular isomer, which they calculated to lie at only slightly higher energy, may contribute the 1320 and 2949 cm−1 absorptions.