Carbonyl 17O Research Articles

Determination of the orientations for the 17O NMR tensors in a polycrystalline l-alanine hydrochloride

We report a solid-state 17O NMR study of the 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors for the carboxyl oxygen in an l-alanine hydrochloride. Using [ 17O]– and [ 13C, 17O]– l-alanine hydrochlorides, both the magnitudes and the orientations in the molecular frame of the 17O EFG and CS tensors could be determined by the analysis of the 17O magic-angle spinning (MAS) and stationary NMR spectra. For the carbonyl oxygen, the smallest EFG tensor component, V XX , and the largest EFG component, V ZZ , roughly lies in the carboxyl molecular plane and the direction of V XX is parallel to the dipolar vector between 13C and 17O, that is, the direction of C O bond. The angles between the intermediate EFG component, V YY , and δ 33 component, and between δ 22 component and V ZZ are found to be approximately 10° and 35°, respectively. We also present the results of the quantum chemical calculations for a theoretical hydrogen-bonding model, indicating that hydrogen-bonding strengths make it possible to vary both magnitudes and orientations of the carbonyl 17O EFG tensors in amino acid hydrochlorides.

A solid-state 17O NMR, X-ray, and quantum chemical study of N-α-Fmoc-protected amino acids

We report the results of a solid-state 17O NMR and X-ray investigation of 17O-enriched N-α-fluoren-9-ylmethoxycarbonyl- l-alanine (Fmoc- l-ALA) and N-α-fluoren-9-ylmethoxycarbonyl- O- t-butyl- l-serine (Fmoc- l-SER). The present X-ray results for Fmoc- l-SER show that the compound crystallized in the monoclinic space group P2 1 with unit-cell dimensions a = 5.843, b = 11.937, c = 15.042 Å, and β = 96.19°. Analysis of 17O magic-angle spinning (MAS) spectra and stationary NMR spectra recorded at multiple magnetic fields of the present Fmoc-protected amino acids yields the magnitudes of hydroxyl and carbonyl 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors with the relative orientations between the two NMR tensors. The 17O quadrupole coupling constants ( C Q) are found to be 7.05–7.60 MHz and 7.90–8.35 MHz, and the spans of the CS tensors are 218–236 ppm and 450–521 ppm, for hydroxyl and carbonyl oxygen atoms, respectively. We also carry out quantum chemical calculations using density functional theory in order to investigate the effects of hydrogen-bond angles on 17O NMR parameters. It is demonstrated that, in addition to the hydrogen bond distances, hydrogen bond angles are an important factor in determining the magnitudes of these tensor components.

Ion Solvation by Channel Carbonyls Characterized by17O Solid-State NMR at 21 T

Recently available ultrahigh magnetic fields offer new opportunities for studies of quadrupole nuclei in biological solids because of the dramatic enhancement in sensitivity and resolution associated with the reduction of second-order quadrupole interactions. Here, we present a new approach for understanding the function and energetics of ion solvation in channels using solid-state 17O NMR spectroscopy of single-site 17O-labeled gramicidin A. The chemical shift and quadrupole coupling parameters obtained in powder samples of lyophilized material are similar to those shown in the literature for carbonyl oxygens. In lipid bilayers, it is found that the carbonyl 17O anisotropic chemical shift of Leu10, one of the three carbonyl oxygens contributing to the ion binding site in gramicidin A, is altered by 40 ppm when K+ ion binds to the channel, demonstrating a high sensitivity to such interactions. Moreover, considering the large breadth of the carbonyl 17O chemical shift (>500 ppm), the recording of anisotropic 17O chemical shifts in bilayers aligned with respect to magnetic field B0 offers high-quality structural restraints similar to 15N and 13C anisotropic chemical shifts.

Evidence of C−H···O Hydrogen Bonds in Liquid 4-Ethoxybenzaldehyde by NMR and Vibrational Spectroscopies

Raman, FTIR, and NMR (both 13C and 17O) spectroscopies are used in a complementary way in order to study the occurrence of C−H···O intermolecular hydrogen bonds in liquid 4-ethoxybenzaldehyde (4EtOB). Additional information concerning the structure of the possible dimers is obtained through ab initio calculations, at the B3LYP/6-31G* level. The strongest evidences of the presence of C−H···O hydrogen bonds in the liquid phase arise from the temperature and solvent intensity dependence of the two bands observed in the νCO region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl 17O NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the νC-H vibrational modes, the 1JCH concentration dependence detected in the NMR spectra, and ab initio results. The experimental observations are consistent with the decrease of the C−H bond length upon hydrogen-bonding, as predicted for the nonstandard blue-shifting hydrogen bonds. Ab init...

Solid-State 17O NMR Investigation of the Carbonyl Oxygen Electric-Field-Gradient Tensor and Chemical Shielding Tensor in Amides

We have presented a systematic experimental and theoretical investigation of the carbonyl oxygen electric-field-gradient (EFG) tensor and chemical shielding (CS) tensor in crystalline amides. Three 17O-labled secondary amides, R1C[17O]-NHR2, have been synthesized: benzanilide (1), N-methylbenzamide (2), and acetanilide (3). Analysis of 17O magic-angle spinning (MAS) and stationary NMR spectra yields not only the magnitude but also the orientation of the carbonyl 17O EFG and CS tensors. For compounds 1−3, the carbonyl 17O quadrupolar coupling constant (QCC) and the span of the chemical shift tensor are found to be in the range of 8.5−8.97 MHz and 560−630 ppm, respectively. The largest 17O EFG component lies in the amide plane and is perpendicular to the CO bond, whereas the smallest component is perpendicular to the N−CO plane. For the carbonyl 17O CS tensor, the principal component with the largest shielding, δ33, is perpendicular to the amide plane, and the tensor component corresponding to the least sh...

A Combined Experimental and Theoretical 17O NMR Study of Crystalline Urea: An Example of Large Hydrogen-bonding Effects

We report the first experimental determination of the carbonyl 17O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R1NHC(O)NHR2. Analysis of magic-angle spinning (MAS) and stationary 17O NMR spectra of crystalline [17O]urea yields not only the principal components of the carbonyl 17O EFG and CS tensors, but also their relative orientations. The carbonyl 17O quadrupole coupling constant (QCC) and the asymmetry parameter (η) in crystalline urea were found to be 7.24 ± 0.01 MHz and 0.92, respectively. The principal components of the 17O CS tensor were determined: δ11 = 300 ± 5, δ22 = 280 ± 5 and δ33 = 20 ± 5 ppm. The direction with the least shielding, δ11, is perpendicular to the CO bond and the principal component corresponding to the largest shielding, δ33, is perpendicular to the NC(O)N plane. The observed 17O CS tensor suggests that, in crystalline urea, the 17O paramagnetic shielding contributions from the σ → π* and π → σ* mixing are greater than t...

A slow rate exchange process allows the detection of both carbonyl (CO) and hydroxyl (OH) 17O nuclear magnetic resonances of the carboxylic group

In this study we report, for the first time, on the detection of both the carbonyl (CO) and the hydroxyl (OH) 17O resonances of the carboxylic group by 17O NMR spectroscopy. Two well separated peaks, at 340.3 and 175 ppm, were detected for the CO and OH groups, respectively, of Boc-[ 17O]Tyr(2,6-diClBzl)-OH in DMSO- d 6. This finding is attributed to the participation of the carboxylic group in a rather strong hydrogen-bonded interaction. These resonances disappeared in the presence of a small quantity of TFA (trifluoroacetic acid), which, however, did not cleave the Boc-group and a broad resonance was detected for both oxygens. In fact, this result indicates that the proposed hydrogen-bonded interaction is not stabilized in the presence of TFA, which is a very strong hydrogen bond breaker. Only one broad signal was observed for both carboxylic oxygens at 251.8 ppm for the HCl[ 17O]Tyr(2,6-diClBzl)-OH in DMSO- d 6 solution, suggesting that the carbonyl oxygen of the Boc-group is probably the proton acceptor group of the carboxylic hydrogen, stabilizing a γ-turn like structure. A single relatively sharp resonance appearing in chloroform, (247.5 ppm) upfield shifted by ∼14.5 ppm, compared to the resonance found in DMSO- d 6 for Boc-[ 17O]Tyr(2,6-ClBzl)-OH in the presence of TFA, is attributed to a γ-turn fraction being in a fast exchange with the open form.

Density functional theory calculations of 17O NMR chemical shifts for substituted trifluoromethyl aryl ketones

A good correlation is found between density functional theory calculated carbonyl 17O NMR chemical shifts and those observed in CCl 4 solution for 16 titled ketones. The calculated values are in good agreement, 0–5 ppm in most cases, with observed ones and show a linear relationship against σ + constants. The implication of these observations is discussed.

Carbonyl 17O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study

Carbonyl 17O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study

Carbonyl17O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study

The effect of a cis-N-methyl group on the carbonyl 17O chemical shift, cis-MSCS, was investigated both from theoretical and experimental points of view in ten amide derivatives. Experimentally, it was observed that the cis-MSCS in N-methylformamide (2) corresponds to a shielding effect of 12.0 ppm with respect to formamide (1). LORG calculations at both the 6–31G* and 6–311G** levels reproduced fairly well this trend, i.e. 10.2 and 11.4 ppm, respectively, provided that as the N-methyl group conformation was such that a C—H bond eclipsed the C—N bond (2a). This is the preferential conformation at the 6–31G*/MP2 level. For other methyl group conformations the LORG calculations did not reproduce that experimental trend. For instance, for an N-methyl C—H bond eclipsing the N—H bond (2b), deshielding cis-MSCSs of 3.7 ppm (6–31G*) and 3.6 ppm (6–311G**) were predicted. Analyses of LORG bond–bond contributions suggested that the interaction that defines 2a as the preferential conformation is an attractive interaction between the in-plane N-methyl C—H bond and the carbonyl oxygen lone pairs. Experimental trends observed for the 17O chemical shifts measured in the remaining compounds can be rationalized on the same grounds.

17O NMR spectroscopy: intramolecular hydrogen bonding in 7-hydroxyindanones

Natural abundance 17O NMR chemical shift data for seven substituted indanones including four hydroxyindanones, three fluorenones including two hydroxyfluorenones, and seven 2-methyleneindanones including four hydroxymethyleneindanones, at 75°C in acetonitrile are reported. The hydroxyindanones, the one hydroxyfluorenone and the hydroxymethyleneindanones capable of intramolecular hydrogen bonding exhibit carbonyl 17O NMR signals which are shielded relative to those incapable of intramolecular hydrogen bonding. The intramolecular hydrogen bonding component (Δδ HB) of the carbonyl 17O NMR chemical shift was determined to be 9.8 ± 1.2 and 10.9 ± 1.4 ppm for the hydroxyindanones and the hydroxymethyleneindanones, respectively. The small Δδ HB values for these hydroxyindanones relative to other ketone systems (about 50 ppm) are discussed in terms of molecular mechanics calculated hydrogen bond geometry.

17O NMR spectroscopy of substituted methyleneindanones: relationship between chemical shift and oxygen atom electron density

Abstract 17O NMR spectroscopic data for eigth β-substituted methyleneindanones obtained at natural abundance in acetonitrile at 75°C are reported. 17O NMR data for ten para-substituted E-benzalindanones, enriched with 17O, were recorded in acetonitrile at 75°C. The 17O NMR data for the E-benzalindanones gave good correlations with sigma plus values, with literature carbonyl IR stretching frequencies, and with literature 17O NMR carbonyl data of chalcones and 5-aryl-2,3-furandiones. The carbonyl oxygen atom electron density (AM1) gave good correlation with the carbonyl 17O NMR chemical shift of both β-substituted methyleneindanones and the E-benzalindanones.

Natural Abundance17O NMR Studies on Methyl N-Arylcarbamates: Torsion Angle and Electronic Effects

Abstract Natural abundance 17O NMR data for 14 substituted methyl N-arylcarbamates obtained in acetonitrile solution at 75°C are reported. The 17O NMR chemical shifts of hindered ortho N-arylcarbamates are shielded relative to unhindered ones; a quantitative relationship is observed between the carbonyl 17O NMR chemical shifts and molecular mechanics (MM2) predicted torsion angles. The carbonyl 17O NMR chemical shifts of meta and para substituted N-arylcarbamates are correlated with Hammett sigma constants.

Oxygen‐17 NMR study of electronic and steric effects in furan‐2,3‐dione derivatives

Abstract17O NMR was used to probe the steric and electronic substituent effects in furan‐2,3‐dione derivatives. The carbonyl 17O chemical shifts correlate with Hammett constants and IR carbonyl stretching band frequencies for 5‐(para‐substituted‐phenyl)furan‐2,3‐diones.

Relaxation mechanisms, bonding, and molecular reorientation in CpRe(CO) 3

Carbonyl 13C and 17O and cyclopentadienyl 13C NMR spin-lattice relaxation times of CpRe(CO) 3 were measured as a function of temperature in the solvent CDCl 3. Analysis of T 1( 13CO) revealed that, unlike in most metal carbonyls, where chemical shift anisotropy is the dominant mechanism, the major contribution to 13C relaxation is via scalar coupling to the directly bonded rhenium. The oxygen-17 quadrupole coupling constant [determined from T,( 17O)], and theirπ★ antibonding orbital population was very close to values reported in several (arene)Mo(CO) 3 complexes, indicating similar pi bond strengths. The C-K force constants ( k co), on the other hand, were quite dissimilar, indicating that k co is not a pure measure of π bonding in metal carbonyls. Comparison of rotational correlation times of the cyclopentadienyl group and M(CO) 3 unit revealed substantial internal rotation of the Cp ring. The rate, however, is slower than expected for afree rotor, indicating the prese

17O NMR spectroscopy: Intramolecular hydrogen bonding in hydroxypyridine carboxy esters

AbstractNatural abundance 17O nmr chemical shift data for 8 aryl esters and 10 pyridine carboxy esters, including 6 ortho‐hydroxy esters, recorded in acetomitrile at 75° are reported. The carbonyl group 17O nmr chemical shift data for methyl 2‐, 3‐ and 4‐pyridinecarboxylate are correlated with σ+ constants. The hydrogen bonding component (ΔδHB) to the ester carbonyl 17O nmr chemical shift for the intramolecular hydrogen bonded ortho‐hydroxy systems are 9.8 ppm, 13.6 ppm and 4.3 ppm for benzoates, 2‐pyridinecarboxylates and 4‐pyridinecarboxylates, respectively. The relationships of the ester ΔδHB values to other hydrogen bond acceptor ΔδHB values are discussed.

Intramolecular Hydrogen Bonds of the CO…︁HO Type as Studied by 17O‐NMR

AbstractThe 17O‐NMR spectra of 1,4‐naphthoquinone and 5‐hydroxy‐1,4‐naphthoquinone (juglone) have been recorded in CDCl3 solution at 40°. In juglone the 17O resonance of the carbonyl peri to the OH group was displaced by 70 ppm to low frequency relative to the resonance in the para‐position. It is shown that this chemical shift arises mainly from intramolecular H‐bonding, the substituent and steric effects being one order of magnitude smaller. Large carbonyl 17O chemical shifts between −34 and −100 ppm were also observed in a series of aromatic aldehydes and ketones where intramolecular H‐bonds of the CO…︁HO type are formed. The H‐bond‐induced carbonyl 17O chemical shifts were linearly correlated with both the 17O and 1H chemical shifts of the OH groups. They represent a most sensitive measure of the strength of intramolecular H‐bonds. The 17O resonances of the OH groups were directed to high frequency on H‐bonding. Analysis of the 17O chemical shifts in 2,2′‐dihydroxy‐benzophenone showed clearly that the two OH groups build H‐bonds simultaneously to the single carbonyl group. The 17O linewidths decreased strongly on H‐bonding; the linewidth of the H‐bonded carbonyl O‐atom in juglone, for example, was reduced by 25% with respect to that of the free carbonyl O‐atom. The carbonyl O‐atom quadrupole coupling constants in juglone, evaluated from the combined use of 13C and 17O relaxation times, were 9.5 and 11.0 MHz, respectively. No correlation was observed between the H‐bond‐induced 17O chemical shifts and the variations in 17O quadrupole coupling constants.

Applications of 95Mo N.M.R-Spectroscopy. XVI. Structure and Bonding in [Mo(Co)3(η6-Arene)] Complexes From 95Mo,13C and 17O Relaxation-Times

Spin-lattice relaxation times (T1) have been measured for the 95Mo, 17O and methine and carbonyl 13C nuclei for a series of [Mo(CO)3(η6-C6H6- nMen)] complexes. In addition to the expected decrease in T1 with increasing molecular volume, the 95Mo and methine 13C data are interpreted in terms of variations in arene -molybdenum bonding. The methine 13C T1 values decrease in the order o- xylene > toluene > p- xylene > m- xylene > mesitylene . The methyl substitution pattern determines the distribution of electron-rich sites on the arene ligand , affecting the molybdenum- arene bond strength and the barrier to arene rotation. These observations support an earlier proposal that downfield 95Mo chemical shifts are associated with increased molybdenum- arene bond strength. 95Mo T1 values are unaffected by the rotational barriers, since they depend only on molecular tumbling perpendicular to the pseudo C3v symmetry axis. While these T1 values decrease as the number of methyl groups increases, an effect due to increasing molecular volume, the order of 95Mo T1 for the xylene complexes is opposite to that of the 13C T1, due to a reduction in the electric field gradient at the 95Mo nucleus. 95Mo quadrupole coupling constants for the series are 1.2�0.2 MHz. The 17O T1 values have also been measured for this series. They are considerably longer, and quadrupole coupling constants smaller (0.87�0.07 MHz), than those of other metallocarbonyls , indicating a significant increase in metal- ligand dπ-pπ - back bonding in the present system. A correlation between carbonyl force constants and 17O quadrupole coupling constants is presented.

Analysis of multinuclear lanthanide-induced shifts. 1. Investigations of some approximations in the procedure for separation of diamagnetic, contact, and pseudocontact shifts

For the 1:1 adducts of La(fod)3, Pr(fod)3, Eu(fod)3, Gd(fod)3, Dy(fod)3, and Yb(fod)3 and adamantanone the 17O, 133C, and 1H bound shifts were determined. With the data set obtained it is demonstrated that a separation of diamagnetic, contact, and pseudocontact shifts gives reliable values for all shift contributions only when a modified procedure is used in the separation. Our data show that diamagnetic and contact shifts are very large for the carbonyl 17O and 13C nuclei. Diamagnetic and contact shifts are also substantial for the remaining 13C nuclei and for 1Hα in the Eu(fod)3 adduct. For all other nuclei the pseudocontact shift is dominant, and a direct comparison of experimental bound shifts with calculated pseudocontact shifts is possible.

Chemical shift correlations in the 13C, 17O and 31P NMR spectra of some Mo(CO) 4((PPh 2O) 2Y(R)R′) (Y(R) = P(O), Si(Me); R′ = alkyl, haloalkyl, aryl) and [Mo(CO) 4(PPh 2O) 2] 2Si complexes

The syntheses and 13C, 17O, 29Si and 31P NMR spectra of a series of Mo(CO) 4((PPh 2O) 2Y(R)R′) (Y(R) = P(O), Si(Me); R′=alkyl, haloalkyl, aryl) and [Mo(CO) 4(PPh 2O) 2] 2Si complexes are given. The chemical shift ranges of the cis and trans carbonyl 13C and 17O, phenyl C(1) 13C and 31P resonances are relatively large and, with the exception of the cis carbonyl 17O chemical shifts, the correlations between the chemical shifts of the various resonances are excellent. These correlations are consistent with the model of metal carbonyl 13C and 17O chemical shifts proposed by Bodner and Todd. In addition they allow the model to be extended to include the diphenylphosphinite 31P chemical shifts in these complexes. The excellent correlations may be due to the presence of the chelate ring which limits the rotation around the molybdenum-phosphorus bond and to the fact that all three groups directly bonded to the phosphorus remains constant.