Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR,LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A₁ = A₂ = 34.4 MHz and A₃ = ?53.7 MHz. The tunneling frequency (3J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.     

Ring and nitrogen inversion in a flexible highly symmetrical molecule: The conformations of 1,3,7,9,13,15,19,21-octaazapentacyclo[19,3,1,1,3,7,19,13,115,19]

The possible conformations of the title compound and their modes of interconversion via ring and nitrogen inversion processes are delineated. At high temperatures (> +80°) the ¹H NMR spectra are consistent with time averaged D_4th symmetry and rapid ring and nitrogen inversion. At lower temperatures (ca. ?10°) the time-averaged symmetry if D_2d and inversion of the 6-membered rings is frozen out, nitrogen inversion remaining rapid. The free energy of activation for the total inversion of all four 6-membered rings is 13·5 kcal mole ^?1, higher than in similar monocyclic systems. This higher energy is a reflection of the multiple ring inversion pathway required for total inversion of all the 6-membered rings.     

NMR study of the inversion at carbon 2 of some 1′3′3′-trimethylindolino disubstituted spirobenzopyranes

The ¹H NMR spectra of some 1′3′3′-trimethylindolino-6,8-disubstituted-spirobenzopyranes in equilibrium with more than 5% of their merocyanine isomer were taken at different temperatures in dimethylsulfoxide d₆. This presents evidence of a fast inversion process at the asymmetric spiro carbon-2. It is shown that the intermediate responsible for the inversion process is certainly not the merocyanine but more likely the open ring cis isomer of the spiropyran.     

27Al ENDOR study of VO2+ adsorbed on δ-alumina

A²⁷ Al ENDOR study on the vanadyl aquoion adsorbed on hydrous δ-alumina is reported. The observed aluminium hyperfine interaction of 7.2 MHz is interpreted in terms of an inner-sphere surface coordination model. The magnitude of the coupling is attributed to spin delocalization from the vanadyl moiety to surface bound aluminium centers. It is concluded that equatorial positions of the VO _aq ²⁺ fragment are occupied by surface functional groups. Proton ENDOR spectra confirm the presence of both axial and equatorial water ligands.     

A proton nuclear magnetic resonance spectroscopic study of conformational equilibration of 3-azabicyclo[3.2.2]nonane

The 251 MHz ₁H NMR spectra of 3-azabicyclo[3.2.2]nonane (1) have been measured from ?40 to ?175°C. Below about ?140°C a conformational process which equilibrates the methylene protons of the CH₂NCH₂ moiety of 1 becomes slow on the NMR time scale. The free-energy of activation (ΔG^#) for this process is 5.9 kcal mol^?1. The results can be interpreted either in terms of ring inversion of the seven-membered ring or limited pseudorotation of the six-membered ring. Possible pathways for effecting conformational equilibration in 1 are discussed.     

Rotation about the CN bond in thioamides: Influence of substituents on the potential function

Barriers to rotation about the CN bond in several unusual thioamides have been studied with 100 MHz ¹H-NMR spectroscopy. This rotation is still frozen at 170°C in p-chlorophenylglyoxylthiomorpholide (2) in DMSO-d₆. NMR spectra and the IR carbonyl frequencies show that 2b and 2c are the most important resonance forms, the latter making a significant contribution to the bonding in the transition state. The high rotational barrier is related to preferential stabilization of the ground state by the opposed orientation of two parallel dipoles (2b). In the sterically crowded molecules 4–(2′-hydroxythiobenzoyl)morpholine (3) and 1–(2′-hydroxythiobenzoyl)piperidine (4) rapid ring inversion between two chair forms of slightly different energy superimposed on the much slower rotation about the CN bond implies that only a mean value for the rotational barrier height can be obtained experimentally.     

Direct observation of 57Fe chemical shifts in diamagnetic organoiron complexes

⁵⁷Fe NMR spectra of 35 organoiron complexes predominantly of the type [Fe(CO)₃(diene)] have been measured by direct detection at 2.9 MHz and at natural isotope abundance (2.2%). The ⁵⁷Fe chemical shift range so far observed is 3000 ppm, corresponding to about 9 KHz. The majority of the resonances are at higher frequency than that of Fe(CO)₅, which is proposed as a secondary standard. The ⁵⁷Fe resonance frequencies are also reported as ratios relative to the ⁷³Ge frequency of GeCl₄.The ⁵⁷Fe shielding is discussed qualitatively in terms of charge distribution in the complexes, very large deshielding effects being observed for cationic olefinic ligands. For η⁴-diene complexes, the shielding also depends upon ligand geometry and decreases with increasing ring size or CCC bond angle. Two types of ferrocenylcarbenium ions may be distinguished, indicated by high- or low-frequency shifts relative to ferrocene. For a number of [Fe(CO)₃(olefin)] complexes ¹J(⁵⁷Fe, ¹³C) coupling constants involving the carbonyl carbon are also reported.     

Spectroscopic investigations of phase transition in 5-chloro-2-pyridone

Spectroscopic investigation of 5-chloro-2-pyridone has been carried out in the temperature range 77–300 K. At room temperature the ³⁵Cl NQR spectrum shows a single line at 35.618 MHz, but at 250.7 K two lines appear at 35.850 MHz and 35.840 MHz respectively indicating the presence of a phase transition. IR, far-IR, laser Raman and dielectric measurements have been carried out to investigate the phase transition further. Low temperature IR studies show splitting of ν(CCl), β(NH) and ν(CO) bands at T_c. Dielectric measurements show a small, but finite, change in the value of the dielectric constant around T_c. Raman spectra at different temperatures support the existence of a new phase, as shown by the appearance of a new band at 81 cm⁻¹, the frequency of which changes slowly as T_c is approached and which disappears at T_c. The temperature dependence of the NQR frequencies has been analysed using Bayer Kushida and Brown equations toevaluate the torsional frequencies.     

Laser magnetic resonance spectra of the PH radical

The PH radical has been detected by laser magnetic resonance spectroscopy at 118.6 μm in the reaction products when hydrogen atoms were passed over red phosphorus. The spectra have been identified as the N = 4 → 5 rotational transition in the ground ³∑^? state and J = 4 → 5 transition in the a¹ Δ state. The hyperfine constants for the ¹Δ state are a_p = 775 MHz and a_H = 28 MHz.     

Conformational disorder in the propagating radical of dimethacrylate polymers

Photopolymerization of methacrylic monomers yields samples with trapped radicals that are easily detected by electron paramagnetic resonance (EPR) spectroscopy. Despite its simplicity, there is no general agreement about the interpretation of this spectrum, in particular, about the role of methylene β protons. An extensive ENDOR study of the propagating radical in photopolymerized dimethacrylates has been carried out in order to obtain detailed information about methylene hyperfine couplings and, thus, about radical conformation. It is shown that literature models are not able to reproduce the ENDOR results and that only accurate fitting of ENDOR spectra obtained by saturating the EPR spectrum at different positions gives reliable information about radical conformation, thanks to the exploitation of conformational selectivity. It turns out that most radicals are in the minimum energy conformation, but any possible conformation is assumed by non negligible fractions of radical.     

Photochemically generated carotenoid radicals on Nafion film and silica gel: an EPR and ENDOR study

Carotenoid cation radicals have been observed at 120 K, by EPR and proton ENDOR measurements, to be formed upon 77 K photolysis of thin films of Nafion or silica gel coated with the carotenoids, β-carotene and canthaxanthin. The powder ENDOR spectra consist of resolvable lines due to couplings larger than 2.8 MHz but smaller than 17 MHz assigned to the methyl protons of the carotenoid cation radical and to an -proton of the planar polyene chain.     

Long range 13C1H coupling constants at bridgehead carbons in some naphthalene and quinoline derivatives, “cross” ring and “through” ring interactions

The ³J_CH couplings at bridgehead carbons in naphthalene and quinoline derivatives have been classified as “through” ring and “cross” ring interactions. Further divisions of the couplings have been introduced in an attempt to rationalize the interactions such that they may be used for structural assignments. The influence of central π-bond order and of the heterocyclic nitrogen atom on the couplings has been discussed. From a study of the ³J couplings at C-9 and C-10 in 45 quinoline derivatives the coupling constant ranges observed were “through” ring (via 9,10-bond): J₉₄ 4.9–6.1 Hz, J₉₅ 5.5–7.3 Hz, J_10.8 4.1–5.3 Hz; “cross” ring: J₉₂ 11.0–13.5 Hz, J₉₇ 6.7–9.8 Hz, J_10.3 4.4–7.3 Hz, J_10.6 5.2–9.1 Hz.     

Spectra and structure of small ring compounds—LX. Raman and infrared spectra,conformational stability,normal coordinate analysis,and ab initio calculations of cyclobutyl acetylene

The Raman spectra (3400 to 10 cm⁻¹ of gaseous, liquid (with qualitative depolarization values) and solid cyclobutyl acetylene, c-C₄H₇CCH, have been recorded. Additionally, the infrared spectra (3500 to 90 cm⁻¹ of the gas and solid have been obtained. The spectra of the fluid phases are consistent with two stable conformers existing at ambient temperature. These data have been interpreted on the basis that the equatorial conformer is more stable than the high energy axial form in both the gas- and liquid-phases, and is the only conformer present in the solid. Two Q-branches are observed in the low frequency vibrational spectra of the gas at 133 and 118 cm⁻¹ and are assigned to the fundamental ring puckering vibration and an associated upper state transition of the low energy equatorial conformer. These data have been used to approximate the form of the potential function governing ring inversion. Experimental values for the enthalpy difference between the two conformers have been determined for both the gas, 282 ± 49 cm⁻¹, and the liquid, 181 ± 15 cm⁻¹, from relative intensities of a pair of Raman lines over 71 and 100°C temperature ranges, respectively. The structure, conformational stability, inversion barrier and vibrational frequencies have been determined by ab initio calculations using the 3-21G and/or 6-31G* basis sets. These calculated results are discussed in comparison to those determined from experiment and to corresponding quantities for some similar molecules.     

Crystal preparation and properties of cesium tin(II) trihalides

Synthesis methods for cesium tin(II) trihalides via aqueous solution and from the melts of anhydrous halides, which ensure freedom from oxidation and the effects of traces of water, are described. The halide compounds CsSnCl₃, CsSnBr₂Cl, CsSnBr₃, CsSnBr₂I, CsSnBrI₂, and CsSnI₃ all have the cubic perovskite structure at elevated temperatures, and all but the first two are good electrical conductors in this form. The growth of single crystals from the melt, and by vapor transport, is outlined.The ${}^{35}\text{Cl}$ nuclear quadrupole resonance spectrum of monoclinic CsSnCl₃ consists of three lines, with frequencies 9,799, 11.005, and 11.695 MHz at 25°C, confirming the presence of pyramidal SnCl₃^? ions in this structure. In CsSnBr₃, there is a single ${}^{81}\text{Br}$ nuclear quadrupole resonance line, with frequency 63.073 MHz at 25°C, which splits into two lines on cooling the sample below 19°C. The low-temperature form of CsSnBr₃ apparently has a tetragonally distorted perovskite structure, with a = 11.59 and c = 11.61 Å at 12°C. A single ${}^{127}\text{I}$ nuclear quadrupole resonance line was observed in the low temperature orthorhombic form of CsSnI₃, with frequency 79.707 MHz at 25°C, and the variation of the frequency of this line with temperature may indicate a minor phase change in CsSnI₃ at 35°C.     

Measurement of excited state population decay time in NH3 by optical adiabatic rapid passage

A strong Q-switched N₂O laser capable of 60 MHz frequency sweep is used to induce population inversion by optical adiabatic rapid passage on the ν₂[asQ(8,7)] transition of ¹⁴NH₃. The fast inversion and the subsequent return to equilibrium are detected by a cw weak counterpropagating N₂O laser, and are used to measure the pressure dependence of excited state population relaxation time T₁ for the infrared transition of interest. this decay is found to be 3.6 times slower than the correpsonding decay in the ground state, an effect which may be explained by the larger inversion energy separation in the excited state of NH₃.     

Electronic spectra of 1- and 9-fluorenecarboxylic acids

The absorption and fluorescence spectra of 1-fluorenecarboxylic acid and its anion have indicated that charge transfer interaction between the COOH group and the fluorene ring is more than that of the COO⁻ group with the ring, both in the S₀ and S₁ states. This has been explained by the greater possible resonance interaction between the aromatic ring and the COOH group than with the COO⁻ group. The respective interactions between the COOH or the COO⁻ group and the fluorene ring in the case of 9-fluorenecarboxylic acid is virtually negligible, both in the S₀ and S₁ states. The COO⁻ group becomes coplanar with the ring when deprotonation takes place at position 9. Various proton transfer reactions have been studied and pK_a values in the S₀ and S₁ states have been determined and discussed. The behaviour of the acids is confirmed by undertaking a similar study with their respective esters.     

EPR and endor studies of cobaloxime(II)

EPR and ENDOR studies were carried out on bis (dimethylglyoximato) cobalt (II) in various frozen solvents. The anisotropic EPR spectra, from which the principal values of the g and ⁵⁹Co hyperfine tensors were determined, enabled orientationally selected ¹H ENDOR spectra to be obtained. In a tetrahydrofuran (THF)/toluene matrix, THF was shown to be axially coordinated to the metal (R_Co-O(THF) = 0.22 nm) with the THF plane at an angle of ≈55° to the yz plane and retention of symmetry about the xz plane. ¹H ENDOR of the dimethylglyoximate ligands yielded information about the H-bond geometry and in-plane assignment of the principal values of the g tensor.     

Optically detected electron—nuclear double resonance and electron—nuclear—nuclear triple resonance study of 17O hyperfine coupling in the lowest nπ* triplet state of benzil

Optically detected ENDOR and electron—nuclear—nuclear triple resonance of ¹⁷O were measured via phosphorescence from ³(nπ^*) benzil in benzophenone-d₁₀ crystals at high magnetic field. The n and π^* spin densities on the oxygen atom are 0.201 and 0.092, respectively, the angle between the two CO bonds being 150°.     

14N nuclear quadrupole resonance in N-acetyl amino acids

¹⁴N Nuclear quadrupole resonance (NQR) spectra of several N-acetyl amino acids and related compounds are reported and analyzed within the framework of the Townes and Dailey theory. The inductive effect of the chloroacetyl group on the nitrogen is discussed. A positive correlation between the π-σ_NC electron density at the nitrogen and the Taft inductive parameter σ* is observed, suggesting that the nitrogen π-charge density in the N-acetyl amino acids does not vary appreciably. NQR data for hippuric acid are compared to the ¹⁴N quadrupole parameters extracted from ENDOR data reported for X-irradiated hippuric acid.     

1H endor investigations in tris-sarcosine-calcium chloride doped with manganese

¹H ENDOR spectra of the ferroelectric tris-sarcosine-calcium chloride doped with Mn²⁺ are reported. All identified protons are located in CH₃ groups of sarcosine molecules. From the ENDOR data, the distances of the CH₃ protons from the manganese ion were determined.