N-lone Pair Research Articles

Enhancing aluminum foil performance in aqueous and organic electrolytes: dual-secure passivation with phthalocyanine as a corrosion inhibitor

The Pc (phthalocyanine) additive prevents Al corrosion by dual-secure mechanisms: electrochemical formation of an AlPc-F layer and physical adsorption of Pc on Al surfaces. Pc contains an N-lone pair, adsorbing on the Al current collector preferentially.

Rotation-Inversion Isomerization of Tertiary Carbamates: Potential Energy Surface Analysis of Multi-Paths Isomerization Using Boltzmann Statistics.

Potential energy surface (PES) analyses at the SMD[MP2/6-311++G(d,p)] level and higher-level energies up to MP4(fc,SDTQ) are reported for the fluorinated tertiary carbamate N-ethyl-N-(2,2,2-trifluoroethyl) methyl carbamate (VII) and its parent system N,N-dimethyl methyl carbamate (VI). Emphasis is placed on the analysis of the rotational barrier about the CN carbamate bond and its interplay with the hybridization of the N-lone pair (NLP). All rotational transition state (TS) structures were found by computation of 1D relaxed rotational profiles but only 2D PES scans revealed the rotation-inversion paths in a compelling fashion. We found four unique chiral minima of VII, one pair each of E- and Z-rotamers, and we determined the eight unique rotational TS structures associated with every possible E/Z-isomerization path. It is a significant finding that all TS structures feature N-pyramidalization whereas the minima essentially contain sp2 -hybridized nitrogen. We will show that the TS stabilities are affected by the synergetic interplay between NLP/CO2 repulsion minimization, NLP→σ* (CO) negative hyperconjugation, and two modes of intramolecular through-space electrostatic stabilization. We demonstrate how Boltzmann statistics must be applied to determine the predicted experimental rotational barrier based on the energetics of all eight rotamerization pathways. The computed barrier for VII is in complete agreement with the experimentally measured barrier of the very similar fluorinated carbamate N-Boc-N-(2,2,2-trifluoroethyl)-4-aminobutan-1-ol II. NMR properties of VII were calculated with a variety of density functional/basis set combinations and Boltzmann averaging over the E- and Z-rotamers at our best theoretical level results in good agreement with experimental chemical shifts δ(13 C) and J(13 C,19 F) coupling constants of II (within 6 %).

Effect of surface functional group over tungsten carbide MXene for efficient NH3 gas sensing using density functional theory

Detecting and sensing NH3 as a pollutant gas is absolute necessity in controlling the excess emission from industries, for indoor air quality supervision, medical and environmental monitoring. 2D materials such as MXene have been explored in the field of gas sensing due to their large surface-to-volume ratio, presence of large number of adsorption sites, mechanical flexibility, rapid charge carrier migration and high conductivity. Present investigation on W2CT2 (T: O, F) includes structural and electronic properties towards NH3 adsorption using density functional theory. Various configurations of termination group, NH3 adsorption over different sites along with different orientations of NH3 molecules considering N-lone pair (LP) and H-1s & O/F-2p orbital interaction have been computed for energetic stability. Values of adsorption energies, charge transfer, charge accumulation over the MXene and formation of interfacial electric field indicate strong physisorption and short recovery time of NH3 gas molecule over W2CT2. Significant change in electrical conductivity after NH3 adsorption, formation of Hydrogen bond between NH3 and MXene and its strength attributes to the variation of electronic, transport and adsorption properties with varying NH3 concentration signifies W2CT2 (T: O, F) as promising 2D material for NH3 gas sensing.

Regioselective Fluorination of 7-Oxo-1,2,4-benzotriazines Using Selectfluor.

7-Oxo-1,2,4-benzotriazines (benzo[1,2,4]triazin-7-ones) are reversible thioredoxin reductase inhibitors that exhibit very strong correlations to pleurotin. In this article, we provide the first synthesis of fluorinated derivatives. Fluorination using Selectfluor of benzo[1,2,4]triazin-7-ones occurs regioselectively and in high yield at the enamine-activated position. This electron N-lone pair activation overrides the activation/deactivation effects of some other substituents. The reaction time was significantly reduced with the use of microwave irradiation at 120 °C and 7 bar. The cytotoxicity and cyclic voltammetry measurements for 8-fluoro-1,3-diphenylbenzo[e][1,2,4]triazin-7(1H)-one (2) are presented and compared with its synthetic precursor, 1,3-diphenylbenzo[e][1,2,4]triazin-7(1H)-one (1a).

Synthetic and Mechanistic Investigation of an Oxime Ether Electrocyclization Approach to Heteroaromatic Boronic Acid Derivatives.

A range of functionalized heteroaromatic boronic acid derivatives are readily accessed by a diboration/6π-electrocyclization sequence. This study revealed the surprising observation that there is a direct relationship between oxime ether stereochemistry and reactivity towards electrocyclization. Specifically, E-oxime ethers are found to be significantly more reactive than their Z-counterparts (stereochemistry relative to azatriene scaffold). In contrast, the configuration at the azatriene alkene terminus has little impact on reaction rates. Computational analysis offers a rationale for this observation; a Nlone pair →C=C π* orbital interaction lowers the energy of the transition state in the electrocyclization of E-oxime ethers. Finally, unreactive Z-oxime ethers can be converted to the corresponding heterocyclic products by a photolytically promoted E→Z isomerization and electrocyclization sequence.

A theoretical investigation of the energetics and spectroscopic properties of the gas-phase linear proton-bound cation-molecule complexes, XCH(+)-N2 (X = O, S).

The structural features, spectroscopic properties, and interaction energies of the linear proton-bound complexes of OCH(+) and its sulfur analog SCH(+) with N2 were investigated using the high-level ab initio methods MP2 and CCSD(T) as well as density functional theory with the aug-cc-pVXZ (X = D, T) basis sets. The rotational constants along with the vibrational frequencies of the cation-molecule complexes are reported here. A comparison of the interaction energies of the OCH(+)-N2 and SCH(+)-N2 complexes with those of the OCH(+)-CO and OCH(+)-OC complexes was also performed. The energies of all the complexes were determined at the complete basis set (CBS) limit. CS shows higher proton affinity at the C site than CO does, so the complex OCH(+)-N2 is relatively strongly bound and has a higher interaction energy than the SCH(+)-N2 complex. Symmetry-adapted perturbation theory (SAPT) was used to decompose the total interaction energies of the complexes into the attractive electrostatic interaction energy (E elst), induction energy (E ind), dispersion energy (E disp), and repulsive exchange energy (E exch). We found that the ratio of E ind to E disp is large for these linear proton-bound complexes, meaning that inductive effects are favored in these complexes. The bonding characteristics of the linear complexes were elucidated using natural bond orbital (NBO) theory. NBO analysis showed that the attractive interaction is caused by NBO charge transfer from the lone pair on N to the σ*(C-H) antibonding orbital in XCH(+)-N2 (X = O, S). The quantum theory of atoms in molecules (QTAIM) was used to analyze the strengths of the various bonds within and between the cation and molecule in each of these proton-bound complexes in terms of the electron density at bond critical points (BCP). Graphical Abstract Linear proton-bound complexes of OCH(+)-N2 and SCH(+)-N2. In these complexes, inductive effect is favored over dispersive effect. The attractive interaction is the NBO charge transfer from N-lone pair of N2 to CH σ* antibonding orbital of XCH(+) (X = O, S).

Proline as a charge stabilizing amino acid in peptide radical cations

Long distance electron transfer in proteins requires relay stations that can be transitorily oxidized or reduced. Although individual prolines cannot assume this function, because of their high ionization energy, it has been shown that polyprolines have the ability to transfer charges. In order to determine the role of the proline in the hole distribution and transport within a PheProPhe tripeptide, the radical cation of a model compound where the phenylalanines carry two or three methoxy groups, respectively, was generated by flash photolysis. Surprisingly, after equilibration, about two thirds of the holes were found to reside on the phen(OMe)2 instead of the more easily oxidizable phen(OMe)3 moiety. DFT calculations showed that, in most of the accessible conformations, the phen(OMe)2• +‐moiety profits more from stabilization by N‐ and/or O‐lone pairs of neighboring amide groups than the phen(OMe)3• + moiety can, which explains the apparently counterthermodynamic hole distribution. Similar calculations showed that, in several conformers of the natural PheProPhe radical cation, the unpaired electron is delocalized over two amide groups, by residing in a σ MO which links the N‐lone pair of the central proline unit with the O‐lone pair of a proximate amino acid, through hyperconjugation via the intervening C―Cα σ‐bond. The same pattern is found in a model compound, N‐acetylproline dimethylamide. It seems that prolines favor conformers which foster hyperconjugation of two amide groups, which lowers the ionization energy of peptides. One should thus consider such interacting amide groups as potential relay stations in the course of electron transfer in polyprolines. Copyright © 2015 John Wiley & Sons, Ltd.

Ruthenium Complexes with Cooperative PNP-Pincer Amine, Amido, Imine, and Enamido Ligands: Facile Ligand Backbone Functionalization Processes

The quantitative formation of enamido complex [Ru(H)PMe(3)(PNP')] (3; PNP' = N(CHCHP(i)Pr(2))(CH(2)CH(2)P(i)Pr(2))) from the reaction of [RuCl(2)PMe(3)(HPNP)] (5; HPNP = HN(CH(2)CH(2)P(i)Pr(2))(2)) with an excess of base (KOtBu) can be explained by beta-hydride migration from an intermediate amido complex [RuClPMe(3)(PNP)] (6; PNP = N(CH(2)CH(2)P(i)Pr(2))(2)). Resulting imine complex [RuCl(H)PMe(3)(PNP*)] (7; PNP* = N(CHCH(2)P(i)Pr(2))(CH(2)CH(2)P(i)Pr(2))) could be independently synthesized and gives 3 with KOtBu. A computational examination of the reversible double H(2) addition and elimination equilibria of enamide 3, amido complex [Ru(H)PMe(3)(PNP)] (1), and amine complex [Ru(H)(2)PMe(3)(HPNP)] (2) explains why [Ru(H)(2)PMe(3)(PNP*)] (8) is not observed experimentally. The distinctly different molecular and electronic structures of related complexes 1 and 3, which feature a Y-shaped distorted trigonal-bipyramid (Y-TBP) for amide 1 but T-shaped TBP for enamide 3, respectively, can be attributed to considerably reduced N-->M pi-donation for the PNP' ligand due to delocalization of the N-lone pair into the unsaturated pincer backbone. The resulting low-lying LUMO of 3 explains its Lewis-acidic behavior, as documented by the formation of octahedral complex [RuH(PMe(3))(2)(PNP')] (14) upon the addition of PMe(3). In comparison, the reaction of 1 with PMe(3) gives a mixture of 2 and 14 via a base-assisted hydrogen elimination pathway. On the other hand, with electrophiles, such as MeOTf, predominant N-methylation is observed for both 1 and 3, producing [RuH(OTf)PMe(3)(MePNP)] (11) and [RuH(OTf)PMe(3)(MePNP')] (12), respectively. This reactivity of 3 contrasts with pyridine-based cooperative pincer analogues and can be attributed to the high flexibility of the aliphatic PNP' pincer ligand. The structural and reactivity patterns place this novel ligand between the parent PNP and aromatic pincer ligands.

On the Origins of Diastereoselectivity in the Alkylation of Enolates Derived fromN-1-(1′-Naphthyl)ethyl-O-tert-butylhydroxamates: Chiral Weinreb Amide Equivalents

The stereochemical outcome observed upon alkylation of enolates derived from N-1-(1'-naphthyl)ethyl-O-tert-butylhydroxamates (chiral Weinreb amide equivalents) may be rationalized by a chiral relay mechanism. Deprotonation with KHMDS leads to a nonchelated (Z)-enolate in which the oxygen atoms adopt an anti-periplanar conformation. The configuration of the N-1-(1'-naphthyl)ethyl group dictates the conformation of the O-tert-butyl group and the configuration adopted by the adjacent pyramidal nitrogen atom. Highly diastereoselective enolate alkylation then proceeds anti to both the bulky tert-butyl group (sterically driven) and the N-lone pair (stereoelectronically driven).

A new rigid cinchona modified (α-IQ) platinum catalyst for the enantioselective hydrogenation of activated ketones: Data to the origin of enantioselection

The enantioselective hydrogenations of ethyl pyruvate (EP), methyl benzoylformate (MBF), ketopantolactone (KPL) and pyruvaldehyde dimethylacetal (PADA) were studied on Pt–alumina catalyst modified by a new modifier namely α-isoquinine (α-IQ) with rigid conformation and for comparison by quinine (Q) in toluene and in acetic acid. The effects of modifier concentration, mixtures of modifiers, hydrogenation of α-IQ and theoretical calculations were examined on the interpretation of features of reactions. Using the Engelhard 4759 catalyst under mild experimental conditions (room temperature, 1 bar hydrogen pressure) the ees were lower in the case of α-IQ than for Q. The inversion of enantioselectivity observed in the case of the previously studied β-isocinchonine (β-ICN) containing C8( R) and C9( S) atoms in toluene as solvent failed to occur in the presence of α-IQ containing C8( S) and C9( R) atoms. Indirect experimental evidence for structure of adsorbed chiral modifier was supported by studies on hydrogenation and relative adsorption strength of α-IQ as well as by theoretical calculations. The significant enantioselectivity changes along the series of otherwise structurally related modifiers (Q, α-IQ and β-ICN) were compared with the ab initio computed geometrical features. The results revealed that besides the effects disclosed up to now in the literature, the orientational angle of the N-lone pair in the quinuclidine moiety relative to the quinoline also influences the structure and the adsorption mode of the intermediate responsible for the enantioselection.

Structural characterization of isomeric 2,3,5‐substituted tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones prepared by cycloaddition of N‐methyl‐C‐arylnitrones to N‐phenyl‐ or N‐methylmaleimide

Abstract1,3‐Dipolar cycloaddition reactions of N‐methyl‐C‐arylnitrones with N‐phenyl‐ or N‐methylmaleimide were studied. The reaction of p‐dimethylamino‐, 4‐benzyloxy‐3‐methoxy‐, p‐nitro‐ and p‐chloro‐substituted phenylnitrones with N‐phenylmaleimide gave cis and trans cycloadducts but that of the corresponding phenylnitrones with N‐methylmaleimides only the cis adducts in the case of p‐dimethylamino and 4‐benzyloxy‐3‐methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by 1H NMR at 233 K and 13C NMR at 208 K) to be mixtures of two invertomers, namely o‐(N‐lone pair antiperiplanar to 3H; minor) and i‐conformations (3H‐C‐C‐3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6–31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.

Nature of cysteine-based Re(V)=O(N2S2) radiopharmaceuticals at physiological pH ascertained by investigation of a new complex with a meso N2S2 ligand having carboxyl groups anti to the oxo group.

X-ray structural characterization of a new isomer of ReO(TMECH3) revealed that it is anti-ReO(DL-TMECH3) (1). (DL-TMECH6 is meso-tetramethyl-ethylene-dicysteine prepared from racemic penicillamine (penH4), the subscript on H indicating the number of dissociable protons; anti denotes the geometric isomer having both carboxyl groups anti to the oxo ligand.) In 1, one carboxyl is deprotonated and coordinated trans to the oxo ligand, and the other is protonated and dangling. The 1H NMR spectrum (assigned by 2D methods) of 1 at pH 4 in aqueous solution revealed that the structure of 1 is the same as in the solid state except for deprotonation of the dangling carboxyl group, affording the monoanion. All chelate ring protons and methyl groups are inequivalent and give sharp signals. As the pH was raised above 7, the 1D 1H NMR signals of the monoanion broadened. Broadening was severe for the methyl and ethylene signals of the tridentate half of the monoanion, and these signals were replaced by new signals for the dianion. The changes suggested a rate process that was intermediate on the NMR time scale, such as CO2- ligation/deligation. With increasing pH the dianion signals sharpened up to pH approximately 8 and then broadened up to pH approximately 10. Finally, the spectrum at pH 10.8 showed only half the number of signals. Each signal was at the midpoint shift between two corresponding signals observed at lower pH, indicating a time averaging between the halves of the DL-TMEC ligand, but no change in protonation state. Two Re=O stretching bands (923 and 933 cm-1) with a constant intensity ratio of approximately 1 were observed for the dianion. These results can be explained if the dianion exists detectably only as a NH-deprotonated/carboxyl-deligated form having two conformers. The conformers differ in the N lone pair (NLp) orientation (either endo or exo with respect to the oxo ligand) and thus have slightly different Re=O stretching frequencies. Although they can be detected by resonance Raman spectroscopy, the conformers are indistinguishable by NMR spectroscopy because NLp inversion (and hence conformer interconversion) is very fast. Interchange of the NH and NLp sites affects the NMR spectra. At pH 8.3 the signals of the dianion are sharpest because interchange is slowest. Below and above pH 8.3, the signals are broader because acid and base catalysis, respectively, increase the rate of interchange between the NH and NLp sites.

Theoretical exploratory study of the strong conformational and diastereotopic effects found in the vertical anion of α-halogen-substituted open-chain and three-membered ring compounds bearing a heteroatom (N, O, P)

A theoretical exploratory comparison of steric effects in the ground state and unstable temporary anion species formed by electron attachment has been achieved for open-chain and three-membered ring compounds bearing O, N, and P atoms. The quantum calculations have been carried out at the MP2/6–311G ∗∗ level. When dealing with open-chain compounds, it is found that a very important enhancement of rotation barriers is found in the anion states. This effect mostly arises from through-space interaction of the odd electron which is predominantly located in σ ∗ CH and σ ∗ XH MOs, concentrated around the H atoms, and the O and N-lone pair electrons. The role of F and Cl substituents is analyzed with respect to unsubstituted compounds. The same type of effect is observed in cyclic compounds. No such strong interactions are present in P-substituted compounds. It is thus emphasized that very strong diastereotopic effects would exist in temporary anionic moieties having high negative EAs. in O and N-substituted series.

Charge-Localizedp-Phenylenedihydrazine Radical Cations: ESR and Optical Studies of Intramolecular Electron Transfer Rates

1,4-Bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)benzene-1,4-diyl (2) its 2,5-dimethyl and 2,3,5,6-tetramethyl derivatives (3 and 4), their radical cations, and bis-radical dications are studied. Crystal structures including those of 2+BPh4-, 32+(BPh4-)2, 4+BPh4-, and 42+(BPh4-)2 establish that ring methylation causes more N-lone pair, aryl π twist without changing the NAr,NAr distance significantly and that both 2+ and 4+ have the charge localized in one hydrazine unit. NMR measurements show that 3+ has about 6% of its spin at the four aryl CH and CMe carbons, while 4+ has about 1.5% of its spin at the four CMe carbons. The average distance between the unpaired electrons of 32+ and 42+ was obtained from the dipolar splittings of their thermally excited triplet states and, as expected, is significantly smaller for 32+ (5.25 Å) than for 42+ (5.63 Å). Rate constants for electron transfer between the hydrazine units of 3+ and 4+ in CH2Cl2 and CH3CN were determined by dynamic ESR. The intervalence radical cations show charge transfer bands corresponding to vertical electron transfer between the ground state and the highly vibrationally excited electron-shifted state, allowing calculation of the parameters controlling electron transfer. Electron transfer parameters obtained from the CT bands using adiabatic energy surfaces which approximate the CT band shapes observed produce rate constants within experimental error of those extrapolated to room temperature from the ESR data for both 3+ and 4+ in both solvents, without using tunneling corrections. The effects of mixing of the electronic wave functions of the reduced and oxidized hydrazine units of 2+ on dNN, the C(t-Bu)N,NA(Ar) twist angle, and the aryl nitrogen lone pair, aryl π twist angle which are observed by X-ray are close to those predicted from the position of the minima on the ET coordinate X of the adiabatic energy surface calculated from the CT band.

Conformational and steric aspects of the inhibition of phenylethanolamine N-methyltransferase by benzylamines.

Compounds of the benzylamine (BA) class are potent inhibitors of phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28). Restriction of the aminomethyl side chain through its incorporation into a cyclic framework as in 1,2,3,4-tetrahydroisoquinoline (THIQ) or 2,3,4,5-tetrahydro-1H-2-benzazepine (THBA) results in enhanced potency as an inhibitor, suggesting a conformational effect in the binding of BAs to the active site; however, these ring systems still retain a high degree of flexibility. We have synthesized a series of conformationally defined analogues of benzylamine in order to probe the effect of conformation, as well as the influence of steric bulk, on PNMT inhibition by this class of ligands. In addition, 1-, 3-, and 4-methyl-substituted THIQs were synthesized and evaluated as flexible models for steric bulk tolerance about this ring system. Substitution by a methyl group on either benzylic position of THIQ results in diminished activity as a PNMT inhibitor; however, 3-methyl-THIQ shows enhanced activity as an inhibitor vs THIQ itself. Full conformational restriction of the BA side chain in analogues 4-8 results in a dramatic loss in inhibitor potency. We attribute this effect to a negative steric interaction between the alkyl bridging units above (or below) the heterocyclic ring systems and an active-site amino acid residue. Conformational restriction of THIQ employing a bridging unit that is not located above (or below) the ring system results in only slightly diminished activity compared to THIQ itself. The relative activities of 4-8 were examined in terms of the conformational descriptors tau 1 and tau 2. Although there is no correlation between tau 1 and activity as a PNMT inhibitor, a qualitative relationship between tau 2 (endo or exo) and activity with PNMT is apparent. We believe that the binding of the N-H and/or N-lone pair of electrons may influence the spatial orientation of these molecules at the active site, resulting in positive binding interactions for compounds 4 and 8 and negative interactions for analogues 5-7. The results from the current investigation are compared to those obtained from a similar study involving conformationally defined amphetamines.

Resolved cis-10-hydroxy-4-n-propyl-1,2,3,4,4a,5,6,10b- octahydrobenzo[f]quinoline : central serotonin stimulating properties.

cis-10-Hydroxy-4-n-propyl-1,2,3,4,4a,5,6,10b -octahydrobenzo[f]quinoline (4) is a centrally acting serotonin (5-HT) receptor agonist of moderate potency. Due to its semirigid character and the obvious similarity between (4aR,10bS)-4 and more potent, centrally acting 5-HT receptor agonist cis-(1S,2R)-8-hydroxy-1-methyl-2-(di-n-propylamino)tetralin (2), we carried out the preparation (via resolution of 6, a precursor of 4) and the pharmacological testing of the enantiomers of 4. We were able to show that the active enantiomers of 4 and 2 coincide in terms of stereochemistry, i.e., that it is the 4aR,10bS enantiomer of 4 that is the more active one. The absolute configuration was assigned on the basis of single-crystal X-ray analysis of the precursor (+)-6 of the active enantiomer (-)-4. Conformational analysis with molecular mechanics (MM2) calculations were performed on the N-methyl analogues of compounds cis-(1S,2R)-2 (cis-(1S,2R)-3) and cis-(4aR,10bS)-4 (cis-(4aR,10bS)-7). Both ammonium and free amine forms were subjected to these calculations. The results show a preference for the N-equatorial conformation, which is corroborated by the X-ray structure of (+)-6.HCl. The relatively low potency of compound cis-(4aR,10bS)-4 might be explained by unfavorable direction of the N-lone pair (or ammonium hydrogen) bond in this compound as compared to cis-(1S,2R)-2 and trans-(4aR,10bR)-5, which can be predicted to be the more active enantiomer of compound 5.

Chemisorbed pyridine on Ni(001): A high resolution electron energy loss study of vibrational and electronic excitations

High resolution angle-resolved electron energy loss spectroscopy has been applied to the study of the vibrational and electronic losses of pyridine chemisorbed on Ni(001). The evaluation of the contributions of in-plane vs out-of-plane vibrational modes of the chemisorbed molecule in the dipole scattering regime shows that orientational phase transitions occur as a function of coverage and temperature. In particular, the molecular plane is parallel to the surface at low coverages at ∼170 K corresponding to a π-bonded species; at higher coverages the molecular plane tilts vertically with respect to the surface which is indicative of a bonding interaction through the N lone pair. Heating to room temperature or room temperature exposures produce an approximately vertically oriented species with some rotation about the N end. This species might be assigned to a chemisorbed α-pyridyl species. The different types of bonding at the surface are manifested in the characteristics of the electronic loss spectra taken with electron primary energies less than 20 eV. A Ni surface state excitation at ∼0.7 eV observed in the clean spectra is quenched when bonding is through the N-lone pair; this can be related to the localization of the surface electronic state. Intramolecular π→π* excitations are not observed when the molecule is π bonded; this is due to a combination of the broadening of the π levels by interaction with the substrate and the screening of the (parallel) dynamic dipole by the metal surface in the dipole contribution to the loss cross section. Off-specular collection geometries show a broad angular distribution for the inelastically scattered electrons as expected for dipole scattering when the loss energy is a significant fraction of the impact energy. A strong charge transfer excitation centered at ∼3 eV is observed for the n-bonded configurations, and this suggests a Ni d→pyridine 3B1 transition which would be maximized in these cases when the overlap between the respective wave functions is greatest. Electronic loss spectra following the thermal evolution of pyridine initially exposed at room temperature show that the molecular species persists up to ∼220 °C, whereupon characteristic electronic losses for C are observed indicating thermal decomposition.

The Analysis of Lanthanoid-induced 13C Shifts for Naphthylamines. Contact Shifts Induced by Gd-chelates and the Spin-delocalization Mechanism

Abstract The 13C shifts of 1- and 2-naphthylamines induced by Gd(fod)3 and Gd(dpm)3 were shown to consist predominantly of a contact contribution, based on the fact that these shifts excellently correlate with the hyperfine coupling constants obtained from the graphical analysis of 13C shifts induced by other Ln(fod)3, where Ln is La, Pr, Nd, Sm, Eu, Dy, Ho, Er, or Yb. It was found from the INDO MO calculations of the spin densities of ligand moieties that these Gd-induced shifts parallel the spin-density distribution of the naphthylmethyl radical better than that of the naphthylamine cation radical and that these shifts are best interpreted in terms of the superposition of the β-spin of the naphthylmethyl radical and the α-spin of the naphthyl radical. This may suggest that a direct spin transfer occurs through space from Gd3+ to the amine molecule, in addition to the normal through-bond delocalization via the N-lone pair.

13C CONTACT SHIFTS OF AROMATIC AMINES INDUCED BY GD(FOD)3 AND SPIN DELOCALIZATION MECHANISM: EVIDENCE FOR THE OCCURRENCE OF DIRECT SPIN TRANSFER

Abstract The 13C contact shift patterns of several aromatic amines induced by Gd(fod)3 have clearly been interpreted by the superposition of σ-type delocalization of positive spin transferred directly into C1 from the Gd ion and π-type delocalization of the negative spin introduced into the N-lone pair orbital.

Visible and PMR spectroscopies of 4-N,N-dimethylammonitrosobenzene and its methyl derivatives

The visible absorption spectra of 4-N,N-dimethylamino- (A), 2-methyl-4-N,N-dimethylamino- (B) and 2,6-dimethyl-4-N,N-dimethylaminonitrosobenzene (C) have been measured. The solvent effects in n → π∗ and π → π∗ transition bands of (A) were examined. It was concluded that the protio solvents and dimethyltin dichloride interact with the oxygen atom of (A), (B) and (C), and that protons interact with both the oxygen and the nitrogen atoms of the N-O group. The PMR spectra of (A), (B) and (C) were measured. The anisotropio effect of the N-lone pair was estimated.