15N NMR Chemical Shifts Research Articles

1H, 13C, 15N NMR and Ab Initio/IGLO/GIAO-MP2 Study of Mono-, Di-, Tri-, and Tetraprotonated Guanidine1

Mono- and diprotonated guanidines were prepared in superacid solutions and studied by 1H, 13C, and 15N NMR spectroscopy. The structures, energies, and NMR chemical shifts were also calculated by ab initio/IGLO/GIAO-MP2 method. Excellent agreement were found between experimental and calculated 13C and 15N NMR chemical shifts. No persistent triprotonated guanidine was observed. Tri- and tetraprotonated guanidines were also studied by ab initio/IGLO/GIAO-MP2 method.

Phosphoric amides.15N NMR study of the P—N bonding in acyclic and cyclic compounds

31P and 15N NMR spectra of 11 cyclic and non-cyclic phosphoramidates were measured. Comparison of the closely related structures demonstrated correlation between the bond angles at nitrogen and the 15N NMR chemical shifts and the 1J(P,N) coupling constants. 15N NMR parameters allowed the exo- and endocyclic nitrogens to be distinguished and could be related to the hydrolytic stability of the P—N bonds. © 1997 John Wiley & Sons, Ltd.

P-N BOND REACTIVITY VS 15N NMR SPECTRISCIOIC CHARACTERISTICS

15N NMR chemical shift values (but not the 1JNP values) are a good indication of the relative basicities of nitrogens in phosphoramidates, as demonstrated by the rates of the acid-catalysed cleavage of the exo- and endocyclic P-N bond in selected substrates.

Density Functional Theory/GIAO Studies of the 13C, 15N, and 1H NMR Chemical Shifts in Aminopyrimidines and Aminobenzenes: Relationships to Electron Densities and Amine Group Orientations

The dependence of the 13C, 15N, and 1H isotropic NMR chemical shifts on amine substitution of aromatic ring systems are examined both experimentally and by DFT/GIAO (density functional theory/gauge including atomic orbitals) methods. There are large, monotonic decreases in the chemical shifts at odd-numbered (ortho and para) pyrimidine ring positions which do not occur at the even-numbered (ipso and meta) atoms as amine groups progressively replace hydrogens at the latter positions. This behavior parallels the computed 2pz electron densities which for the pyrimidine series increase monotonically at N1, N3, and C5 but exhibit small changes at the C2, C4, and C6 positions. Identical trends are noted for the aminobenzenes. The ring atom chemical shifts and 2pz electron densities at ortho and para (but not meta) positions are quite sensitive to the orientations of the amine groups which are pyramidalized as the result of balance between delocalization with the ring and the use of strongly directed sp3 orbital...

Heterocycles as donor and acceptor units in push–pull conjugated molecules. Part 1

The synthesis and spectroscopic investigation of a number of push–pull ethenes in which the donor moiety is represented by a π-excessive five-membered heterocycle (pyrrole, indole and thiophene) and the acceptor group is a π-deficient heterocyclic azine ring (pyridine, pyrazine, pyrimidine, pyridazine) are described. The intramolecular charge transfer in both the neutral compounds and the corresponding N-alkylpyridinium triflates is discussed and confirmed on the basis of three different descriptors, ΔλHetPh,Δλ+n, and Δλsolv1solv2, that take into account the substitution of a phenyl with a heterocyclic donor ring, charge effects and solvatochromism, respectively. According to the ΔλHetPh descriptor, the intramolecular charge transfer in the described diheteroarylethenes increases upon increasing the electron-withdrawing capacity of the acceptor, sustained by the presence of either more than one nitrogen atom or the positive charge in the heterocyclic azine. The described pyridinium derivatives belong to the rarely investigated class of dimethine cyanine dyes. The response of the 13C and 15N NMR chemical shift data appears to be less clear because of the low sensitivity of the NMR probes to remote substitution. © 1997 John Wiley & Sons, Ltd.

Heterocycles as donor and acceptor units in push-pull conjugated molecules. Part 1

The synthesis and spectroscopic investigation of a number of push–pull ethenes in which the donor moiety is represented by a π-excessive five-membered heterocycle (pyrrole, indole and thiophene) and the acceptor group is a π-deficient heterocyclic azine ring (pyridine, pyrazine, pyrimidine, pyridazine) are described. The intramolecular charge transfer in both the neutral compounds and the corresponding N-alkylpyridinium triflates is discussed and confirmed on the basis of three different descriptors, ΔλHetPh,Δλ+n, and Δλsolv1solv2, that take into account the substitution of a phenyl with a heterocyclic donor ring, charge effects and solvatochromism, respectively. According to the ΔλHetPh descriptor, the intramolecular charge transfer in the described diheteroarylethenes increases upon increasing the electron-withdrawing capacity of the acceptor, sustained by the presence of either more than one nitrogen atom or the positive charge in the heterocyclic azine. The described pyridinium derivatives belong to the rarely investigated class of dimethine cyanine dyes. The response of the 13C and 15N NMR chemical shift data appears to be less clear because of the low sensitivity of the NMR probes to remote substitution. © 1997 John Wiley & Sons, Ltd.

Coloration of Aromatic Polyimides and Electronic Properties of Their Source Materials

The relationships between the color intensities of polyimide films and the electronic properties of their source materials—aromatic diamines and aromatic tetracarboxylic dianhydrides—are discussed. The arrangement of the diamine moieties in order of color intensity of polyimides shows fairly good agreement with the order of the electron-donating properties of the diamines estimated from 15N NMR chemical shifts (δN). On the other hand, the arrangement of the dianhydrides moieties in order of color intensity of polyimides agrees with the order of the electron-accepting properties of the dianhydrides estimated from experimental and calculated electron affinity (EA) although systematic inconsistencies are observed for the dianhydrides having-CF3 groups and a benzophenone carbonyl group. These results are consistent with the formation of charge transfer complex (CTC) and indicate that the electron-donating properties of diamines and electron-accepting properties of dianhydrides are retained to a significant extent even in polyimide molecular chains.

Synthesis and spectroscopic characterization of (trialkll/triaryl)-phosphine gold(I) thiocyanate complexes

R 3PAuS 13C 15N (where R 3P = Me 3P, Et 3P, i-Pr 3P, p-TolPh 2P, Ph 3P and cyanoethyl-phosphine CEP) complexes have been prepared. The AuNCS/AuSCN ratios for these complexes have been determined, using IR spectroscopy ( v CN integrated absorption intensity ratios). The observation that the AuNCS/AuSCN ratio increases as the trans influence of the phosphine ligand increases is completely in agreement with the trend predicted by the antisymbiotic trans influence theory. In the solid state only the S-bonded isomer of each complex is observed. The 13C, 15N and 31P NMR chemical shifts of these complexes were also measured and are compared with R 3PAu 13C 15N complexes. The results show that unlike R 3PAu 13C 15N complexes, which undergo disproportion in solution, the R 3PAuS 13C 15N complexes do not undergo disproportion.

Correlation of 15N NMR Chemical Shifts with Calculated Partial Charges in Pyridine N-Oxides

Correlation of 15N NMR Chemical Shifts with Calculated Partial Charges in Pyridine N-Oxides

ORB, a homology-based program for the prediction of protein NMR chemical shifts

A computer program (ORB) has been developed to predict 1H,13C and 15N NMR chemical shifts of previouslyunassigned proteins. The program makes use of the information contained in achemical shift database of previously assigned proteins supplemented by astatistically derived averaged chemical shift database in which the shifts arecategorized according to their residue, atom and secondary structure type[Wishart et al. (1991) J. Mol. Biol., 222, 311–333]. The predictionprocess starts with a multiple alignment of all previously assigned proteinswith the unassigned query protein. ORB uses the sequence and secondarystructure alignment program XALIGN for this task [Wishart et al. (1994)CABIOS, 10, 121–132; 687–688]. The prediction algorithm in ORB isbased on a scoring of the known shifts for each sequence. The scores dependon global sequence similarity, local sequence similarity, structuralsimilarity and residue similarity and determine how much weight one particularshift is given in the prediction process. In situations where no applicablepreviously assigned chemical shifts are available, the shifts derived from theaveraged database are used. In addition to supplying the user with predictedchemical shifts, ORB calculates a confidence value for every prediction. Theseconfidence values enable the user to judge which predictions are the mostaccurate and they are particularly useful when ORB is incorporated into acomplete autoassignment package. The usefulness of ORB was tested on threemedium-sized proteins: an interleukin-8 analog, a troponin C synthetic peptideheterodimer and cardiac troponin C. Excellent results are obtained if ORB isable to use the chemical shifts of at least one highly homologous sequence.ORB performs well as long as the sequence identity between proteins with knownchemical shifts and the new sequence is not less than 30%.

Correlation of 13C- and 15N-NMR Chemical Shifts of Ylidic Carbon and Nitrogen with Calculated Partial Charges in Pyridinium Dicyanomethylides

Correlation of 13C- and 15N-NMR Chemical Shifts of Ylidic Carbon and Nitrogen with Calculated Partial Charges in Pyridinium Dicyanomethylides

Sum-over-states density functional perturbation theory: Prediction of reliable 13C, 15N, and 17O nuclear magnetic resonance chemical shifts

Sum-over-states density functional perturbation theory (SOS-DFPT) has been used to calculate 13C, 15N, and 17O NMR chemical shifts of 20 molecules, for which accurate experimental gas-phase values are available. Compared to Hartree–Fock (HF), SOS-DFPT leads to improved chemical shift values and approaches the degree of accuracy obtained with second order Mo/ller–Plesset perturbation theory (MP2). This is particularly true in the case of 15N chemical shifts where SOS-DFPT performs even better than MP2. Additional improvements of SOS-DFPT chemical shifts can be obtained by empirically correcting diamagnetic and paramagnetic contributions to compensate for deficiencies which are typical of DFT.

Nitrogen-15 NMR studies of hydrogen bonding and proton transfer in complexes of pyridineN-oxides with dichloroacetic acid in CDCl3

The 15N NMR chemical shifts of eight substituted pyridine N-oxides (B) and their complexes (AHB) with methanol and dichloroacetic and trifluoromethanesulfonic acids were measured in chloroform-d at the natural abundance level. The measured chemical shifts are strongly affected by substituents, hydrogen bond strength and protonation. The plot of the relative chemical shifts, Δδ(15N) = δ(AHB) − δ(B), against ΔpKa gives a titration curve that reflects the variation of the hydrogen bond strength and the proton moves from the acid to the N-oxide. According to previous IR data, in the complexes from the inversion region (ΔpKa = 1·26), the proton is either delocalized (B…H…A) or more likely the lifetime is so short (⩽10−13 s) that the method does not recognize the B…H-A and B+H…A− species. Protonation of pyridine N-oxides shifts the nitrogen signal ca 50 ppm to lower frequencies compared with the ca 123 ppm for pyridines.

Prospects for resonance assignments in multidimensional solid-state NMR spectra of uniformly labeled proteins.

The feasibility of assigning the backbone 15N and 13C NMR chemical shifts in multidimensional magic angle spinning NMR spectra of uniformly isotopically labeled proteins and peptides in unoriented solid samples is assessed by means of numerical simulations. The goal of these simulations is to examine how the upper limit on the size of a peptide for which unique assignments can be made depends on the spectral resolution, i.e., the NMR line widths. Sets of simulated three-dimensional chemical shift correlation spectra for artificial peptides of varying length are constructed from published liquid-state NMR chemical shift data for ubiquitin, a well-characterized soluble protein. Resonance assignments consistent with these spectra to within the assumed spectral resolution are found by a numerical search algorithm. The dependence of the number of consistent assignments on the assumed spectral resolution and on the length of the peptide is reported. If only three-dimensional chemical shift correlation data for backbone 15N and 13C nuclei are used, no residue-specific chemical shift information, information from amino acid side-chain signals, and proton chemical shift information are available, a spectral resolution of 1 ppm or less is generally required for a unique assignment of backbone chemical shifts for a peptide of 30 amino acid residues.

Kinetic Analysis of the Stepwise Platination of Single‐ and Double‐Stranded GG Oligonucleotides with Cisplatin and cis‐[PtCl(H2O)(NH3)2]+

AbstractWe report the first direct comparison of the kinetics of platination of defined single‐ and double‐stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14‐mer duplex d(A‐T‐A‐C‐A‐T‐G‐G‐T‐A‐C‐A‐T‐A)·d(T‐A‐T‐G‐T‐A‐C‐C‐A‐T‐G‐T‐A‐T) with [15N]cisplatin and cis‐[PtCl(H2O)‐(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis‐[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt‐NH3, and is very long‐lived (persists for >5 d). The Pt‐Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long‐lived monofunctional adducts of platinum drugs with DNA are discussed.

Nitrogen-15 NMR chemical shifts in oligopeptides coordinated to cobalt(III)

Dipeptide, tripeptide, and tetrapeptide complexes with cobalt(III) ions were studied as model compounds for evaluation of 15N NMR chemical shifts induced in proteins upon binding transition metal ions. Coordination of oligopeptides to cobalt(III) resulted in large negative 15N NMR shifts for amine nitrogens (− 76 to − 32 ppm) and deprotonated amide nitrogens (− 47 to −10). Coordination-induced shifts were affected by the nature of moiety at the trans position; the shifts were always larger with a carboxylato oxygen than with an amine nitrogen in the trans position. Thus, coordination-induced 15N NMR shifts provided direct and specific information on the stereochemistry of peptide coordination. Two new complexes, [Co(Gly-gly-gly-glyH −3)(NH 3) 2] and Ba[Co(Gly-L-hisH −2)(NO 2) 3], were synthesized and their structure was determined by NMR spectroscopy.

Hydrogen/deuterium isotope effects on the 15N NMR chemical shifts and geometries of low-barrier hydrogen bonds in the solid state

In this study, hydrogen/deuterium isotope effects on the geometry and the 15N NMR chemical shifts of strongly hydrogen bonded, solid hydrogen-bisisocyanide salts of the type [MCN⋯L⋯NCM] −X + 1 (L  H, D) are described both experimentally and theoretically. The theoretical studies include PM2 6–31+ G(d,p) ab initio calculations of geometries and subsequent calculations of the proton and the deuteron vibrational states in the hydrogen bond of the model compounds [CN⋯L⋯NC] −Li + 1a (L  H) and 1b (L  D). The Li +, at various fixed CLi distances, represents the external electric field. Furthermore, the 15N NMR chemical shifts were calculated using the Individual Gauge for Localized Orbitals method. The calculated isotope effects depend strongly on the asymmetry of the hydrogen bond caused by the influence of the Li + cation. These results are compared with those of a solid state 15N cross polarization, magic angle spinning NMR study of the metal-stabilized salts 1c and 1d, where M  Cr(CO) 5, X +  AsPh + 4 and L  H or D, as well as of those of 1e and 1f, M  Cr(CO) 5, and X +  NnPr + 4. In the case of 1c and 1d the hydrogen bond is symmetric and isotope effects on the 15N NMR chemical shifts are not observed, as predicted theoretically. By contrast, and in agreement with the calculations, large isotope effects are observed for 1e and 1f, where the hydrogen bond symmetry is lifted by a strong interaction with the counterion. So far, solid state hydrogen/deuterium isotope effects on the NMR chemical shifts of hydrogen bonded nuclei have, to our knowledge, not been observed and may be used as a promising novel tool in hydrogen bond research.

Ab Initio Study of Pernitric Acid: Comparison with Experimental Spectra

A high-level ab initio theoretical study has been performed on the spectra and bond energies of pernitric acid, HO2NO2. Excellent agreement with experimental rotational constants, 15N NMR chemical shifts, HO2 + NO2 reaction enthalpy, and vibrational frequencies are obtained. Slight revisions of low-frequency normal modes have been made. Vertical excitations are computed by CASPT2 for comparison with future high-resolution UV−visible spectra and match the low resolution UV−visible spectra of pernitric acid in solution. The optimized geometries reported are superior to previous microwave structures because assumptions of bond lengths and angles from the analogous nitric acid and hydrogen peroxide molecules introduce significant errors.

Correlation of Electronic Effects in N-Alkylnicotinamides with NMR Chemical Shifts and Hydride Transfer Reactivity.

The (13)C and (15)N NMR chemical shifts for ring atoms of a series of N-alkylnicotinamides are shown to be correlated with their reduction potentials and reactivities toward NaBH(3)CN. The nicotinamide compounds include N-ethyl-N-benzyl-N-[p-(trifluoromethyl)benzyl]-, N-(p-cyanobenzyl)-, N-(carbomethoxymethyl)-, and N-(cyanomethyl)nicotinamides. The values of delta()13(C) for all the ring carbons increase with increasing electron-withdrawing power of the N-alkyl substituent. The value for C-4 increases the most, a range of 2.4 ppm in this series, whereas those for other atoms increase on the order of 1 ppm. The value of delta()15(N) for N-1 decreases with increasing electron-withdrawing power over a range of 20 ppm. The NMR data indicate that inductive electron withdrawal by N-alkyl substituents polarizes the pi-electron system to decrease electron density on ring carbons and increase electron density on the ring nitrogen. The values of log k (second order) for reduction of these compounds by NaBH(3)CN are proportional to the values of delta()13(C) for C-4 and inversely proportional to delta()15(N) for N-1. The reduction potentials are proportional to delta()13(C). The substituent effects are qualitatively similar to the substrate-induced electrostatic effects on the nicotinamide ring of NAD(+) at the active site of UDP-galactose 4-epimerase (Burke, J. R.; Frey, P. A. Biochemistry 1993, 32, 13220-13230). However, they differ quantitatively in that the upfield perturbation at N-1 is smaller in the enzyme and the signal for C-6 is also shifted upfield. The substrate-induced enzymatic perturbation of electron density at C-4 of NAD(+) quantitatively accounts for its increase in reactivity at the active site, but the perturbation at N-1 is less closely correlated with reactivity.

Ortho‐Substituent effects in N‐arylacetamides. NMR and molecular mechanics investigation

Abstract1H, 13C, 15N and 17O NMR spectra of N‐phenylacetamide (acetanilide) and 21 ortho‐substituted acetanilides were measured and assigned. The observed NMR parameters are related to the Hammett substituent parameters and conformational characteristics of the acetamido moiety estimated by molecular mechanics calculations. Significant relationships were found for the 13C NMR chemical shifts of C‐5 (para to substituent) and the direct spin–spin coupling constant, 1J(C, H), of C‐3 (ortho to substituent) with Hammett substituent parameters. For 15N NMR chemical shifts of the amido nitrogen, no general correlation with the Hammett substituent parameters was found. The interactions between functionalities contiguous to the carbonyl group and the amino nitrogen in acetanilides are effectively hampered owing to the increased twist angle between the planes containing the phenyl ring and the H–N–C fragment in derivatives bearing bulky ortho substituents. Especially in ortho‐halogen‐substituted derivatives the 15N NMR chemical shift of the amino nitrogen is very clearly related to the twist angle between the phenyl ring and H–N–C fragment. For 17O NMR chemical shifts of the acetamido carbonyl, the observed variations can be related to the steric inhibition of resonance between the benzene ring and the acetamido group.