Assignment of the Phosphorus,Proton and Carbon-NMR Spectra of [(Tetrahydro-2-hydroxy-5,5-dimethyl-4H-1,4,2-oxazaphosphorin-4-yl)methyl] phosphonic acid P-Oxide

The ¹H and ¹³C nmr signals of the methylene groups directly linked to the phosphorus atoms, namely P(2)-C(3)H₂ and P(8)-C(7)H₂ in 1a were assigned with the help of hetronuclear decoupling experiments and HETCOR spectrum.     

Further studies on the structural determination of esters of 4-Oxo-4H-1-heiizopyran-2-carboxylic acids using the paramagnetic shift reagent eu(FOD)3

The effect of Eu(fod)₃ on the signals of benzylic ruethvlene protons in the pmr spectra of 4-oxo-4H-1-benzopyran derivatives is described. Assignments were confirmed by decoupling experiments. The technique provides a method for distinguishing between structural isomers.     

Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery

Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Herein, we demonstrate that electron decoupling with chirped microwave pulses enables improved observation of DNP‐enhanced ¹³C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. We report the first measurement of electron longitudinal relaxation time (T_1e) during magic angle spinning (MAS) NMR by observation of DNP‐enhanced NMR signals (T_1e=40±6 ms, 40 mM trityl, 4.0 kHz MAS, 4.3 K). With a 5 ms DNP period, electron decoupling results in a 195 % increase in signal intensity. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improve the sensitivity of ¹³C in the vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS‐NMR transients being governed by short electron T₁ and fast DNP transfer.     

Selektive Verbreiterung von 1H-NMR Restsignalen deuterierter Lösungsmittel

¹H {²H} off-resonance noise decoupling experiments with incompletely deuterated compounds result in a selective broadening of those residual proton signals which are split by H/D couplings. By this technique hidden resonance lines which are overlapped by solvent signals can be easily detected, as is demonstrated by the analysis of acetone/acetone-d₆ mixtures. Typical experimental details are given.     

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S₁ state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.     

The assignment of the 1H and the 13C NMR chemical shifts of N-phenyl-2,4-dimethylbuta-1,3-diene-1,4-sultam

The assignment of the signals in the ¹³C and ¹H NMR spectra of N-phenyl-2,4-dimethylbuta-1,3-diene-1,4-sultam is difficult for the signal pairs C-2 and C-4, C-1 and C-3, (C-1)? H, (C-2)? CH₃ and (C-4)? CH₃. The ¹³C NMR spectrum recorded under gated decoupling conditions provide long-range couplings which make possible an unambiguous assignment of the ¹³C NMR signal pairs. Application of the ¹H CW off-resonance decoupling technique in recording the ¹³C NMR spectra enables the assignment information from the ¹³C NMR spectrum to be transferred to the ¹H NMR spectrum.     

Coalescence of magnetically non-equivalent proton signals in NMR spectra and spin decoupling by torsional transitions

The complex effect of torsional librations on high resolution NMR spectra, which can coalesce non-equivalent proton signals with relatively high Δν_AB and also decouple spin systems by averaging the chemical shift in excited torsional states, is determined and analyzed.     

Natural abundance 13C NMR investigation of substituted bromobenzene using noise decoupling of proton resonances

The natural abundance ¹³C NMR spectra of bromobenzene and ten derivatives have been obtained using the technique of noise modulated decoupling of the proton resonances. Assignments have been made for all ¹³C resonance signals using an additivity rule. The chemical shifts of the aromatic carbon nuclei in the para-substituted compounds are discussed in terms of the Taft parameters (σ_R, σ_I) and the electronegativity of the substituent.     

Photosensitized charge injection into TiO2 particles as studied by microwave absorption

The fluorescence of rose bengal is quenched with 96% efficiency when this dye is adsorbed on TiO₂ particles. Microwave measurements on suspensions of these dye-coated particles have shown large absorption signals when the suspension is irradiated with pulses of 532 nm light from a laser. The microwave absorption is attributed to mobile charge carriers injected into the TiO₂ from the excited singlet state of the dye. The observed rise time of the signal indicates no intermediate surface state of greater than 2 ns lifetime. Various characteristics of the observed signals are compared with those from TiO₂ single crystals.     

The radio-frequency electric resonance technique—a tool for investigating ion-atom collisions

The rf electric resonance technique is discussed with special regard to its applicability to the analysis of excitation processes in ion-atom collisions. In particular, it is useful for investigating substate populations of excited Helium atoms and other He-like systems. A convenient set-up for the rf resonance circuit and interaction region is described. Within the frequency range 100 MHz?ν?1000 MHz, rf electric fieldsE ₁?100 V/cm were applied to He atoms excited by ion impact.E ₁ can accurately be measured using level-crossing techniques, and thus a detailed analysis of the shapes and amplitudes of resonance signals becomes possible.     

Microwave spectral emission from a glow discharge-filled 18–26 GHz fabry-perot cavity spectrometer

A Fabry-Perot semiconfocal cavity resonator containing air at reduced pressure forms part of a Stark-modulated microwave spectrometer operated near the water 22.235-GHz (6₁₆–5₂₃) and ammonia 23.870 GHz (J = 3, K = 3) absorption lines. A glow discharge is formed in the cavity by an 8.333-kHz sinusoidal electric field of strength up to 140 kV m^?1. Under these conditions in the pressure region near 0.01–1 mbar, microwave emission from water and ammonia is observed when microwave energy between 4 to 6 dBm (source output power) at frequencies near that of the line is coupled into the cavity. Spectral features indicate that the emitting molecules arise from recombination of H + OH and H + NH₂. Predischarge features indicate that H₂O is excited directly by electron impact with simultaneous dissociation to OH and its excitation. NH₃ appears to be excited indirectly by energy transfer possibly from a metastable state resulting in dissociation to radicals in the discharge. Significant signal/noise treatment of the signals is found, compared with signals arising from coventional rotational (H₂O) or inversion (NH₃) absorption at these lines.     

Ultrafast dynamics in ammonia clusters: analysis of protonated and unprotonated cluster ion signals

The dynamics of ammonia clusters excited to the à state with 160 fs laser pulses of 6.2 eV was studied by pump-probe experiments with a low probe photon energy of 3.1 eV. Protonated as well as unprotonated cluster ion signals have been observed. The time evolution of both species is characteristic of the intermediate rearrangement and fragmentation processes. The observations strongly support a previously developed kinetic model for this dynamics with the signal at long delay times>6 ps reflecting the species involved in the absorption dissociation ionization (ADI) mechanism. Strong evidence is found for the formation of an internally ‘quasi protonated’ excited state and of ammoniated NH₄ radicals.     

Spectral editing for 13C NMR signals of deuterated compounds

J-modulation of ¹³C spin echo signals is used to develop a strategy for the analysis of deuterated compounds. On the basis of ¹ J(¹³C²H) and an experimental set-up that allows both ²H and ¹H decoupling, ¹³C subspectra for various groups can be produced. The application of difference methods facilitates the analysis and, finally, leads to the identification of CD, CD₂, CD₃, CHD, CHD₂ and CH₂D groups.     

An 1H-NMR. Spectroscopic Study of Alloxazines and Isoalloxazines

¹H-NMR.-spectra of a series of alloxazines and isoalloxazines and certain cationic derivatives were investigated (Tab. 2 and 5). Unequivocal assignment of all resonance signals was achieved in some compounds by selective deuteriation also by double resonance technique. The coupling constants were verified by computer simulation. Considerable enhancement of the signals due to H-C(9) and H-C(6) is found on decoupling of H₃C-C(7), H₃C-C(8) and H₃C-N(10), resp. These results are compared with those obtained with FAD. The methyl resonance signal of the H₃C-C(7) compounds give doublets due to coupling with H-C(6). The difference in chemical shifts observed upon successive formal introduction of methyl groups into the benzene nucleus of (iso)alloxazines indicates that the molecule becomes less planar thereby. The pyrimidine ring of (iso)alloxazines does not contribute to the ring current except by indirect effects via the carbonyl groups. The experimental data are compared with published MO calculations and discussed.     

NMR artifacts caused by decoupling of multiple‐spin coherences: improved SLAP experiment

Contrary to common expectations, multiple‐spin coherences containing products of proton and heteronucleus operators (e.g. H_uC_x, u = x, y, z) can produce not only sidebands but also noticeable centerband NMR signals of the heteronucleus during acquisition under ¹H broadband decoupling. Such centerband signals of low abundant heteronuclei can be sources of relatively strong unexpected artifacts in NMR experiments that aim to detect very weak signals from much less‐abundant isotopomers, e.g. ¹³C–¹³C ones. These findings lead to a new design of Sign Labeled Polarization Transfer (SLAP) pulse sequence (MSS‐SLAP) with improved suppression of centerband peaks that are because of singly, e.g. ¹³C, labeled molecules (parent peaks). The MSS‐SLAP experiment and its MSS‐BIRD‐SLAP variant are compared with a few older SLAP versions. Copyright © 2015 John Wiley & Sons, Ltd.     

Counterpoise corrections to the evaluation of the bimolecular interaction energy components

Counterpoise corrections to the coupling terms of the bimolecular interaction energy decomposition are introduced and examined on a set of electron donor-acceptor dimers X·BH₃ (X=H₂O, NH₃, PH₃, LiH, CO). The interaction energy decomposition of Kitaura and Morokuma, and the decoupling of E _MIXsuggested by Nagase Fueno, Yamabe and Kitaura have been employed.Counterpoise corrections have numerical influence on two terms only of the NFYK decoupling. This decoupling gives useful additional information on the nature of the chemical interaction when applied to STO-3G minimal basis set wavefunctions, but fails when applied the 4-31G wavefunctions.     

Heteronuclear dipolar decoupling in liquid‐crystal NMR using supercycled SWf‐TPPM sequences

Heteronuclear dipolar decoupling is an essential requirement for extracting structural information from the ¹³C NMR spectra of liquid crystals. Efficient schemes for heteronuclear dipolar decoupling in such systems are formulated here by supercycling SW_f‐TPPM, a sequence introduced recently for this purpose in rotating solids. These sequences are compared with two other commonly used decoupling schemes in liquid‐crystal NMR, SPINAL‐64 and SW_f‐TPPM, by analyzing the intensities of various resonances in the proton decoupled ¹³C spectrum of the liquid‐crystal 4‐n‐pentyl‐4′‐cyanobiphenyl (5CB). The effectiveness of the decoupling programs with respect to experimental parameters such as RF field strength, decoupler offset frequency and phase angle is also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

Electron–Proton Decoupling in Excited‐State Hydrogen Atom Transfer in the Gas Phase

Hydrogen‐release by photoexcitation, excited‐state‐hydrogen‐transfer (ESHT), is one of the important photochemical processes that occur in aromatic acids and is responsible for photoprotection of biomolecules. The mechanism is described by conversion of the initial state to a charge‐separated state along the O(N)‐H bond elongation, leading to dissociation. Thus ESHT is not a simple H‐atom transfer in which a proton and a 1s electron move together. Here we show that the electron‐transfer and the proton‐motion are decoupled in gas‐phase ESHT. We monitor electron and proton transfer independently by picosecond time‐resolved near‐infrared and infrared spectroscopy for isolated phenol–(ammonia)₅, a benchmark molecular cluster. Electron transfer from phenol to ammonia occurred in less than 3 picoseconds, while the overall H‐atom transfer took 15 picoseconds. The observed electron‐proton decoupling will allow for a deeper understanding and control of of photochemistry in biomolecules.