Ned H. Martin

An algorithm for predicting the NMR shielding of protons over substituted benzene rings.

In a strong magnetic field, hydrogen nuclei located over an aromatic ring experience a reduced magnetic field as a result of the induced magnetic field associated with circulating pi electrons. We used GIAO-SCF, an ab initio subroutine in Gaussian 94 to calculate isotropic shielding values and to determine the proton nuclear magnetic resonance (NMR) shielding increment for a simple model system: methane held at various positions over a substituted benzene ring. The NMR shielding increments experienced by the proximal protons of methane have been mapped as a function of their position X, Y, and Z relative to the center of aniline and, separately, nitrobenzene. A mathematical function of the same form has been fit to the three-dimensional shielding increment surface at each of five distances from the face of each aromatic ring. In addition, a single mathematical equation has been developed for predicting the shielding caused by either substituted aromatic ring. The chemical shifts predicted by using the results of this equation in conjunction with additive substituent increments are compared to observed values.

An Improved Model for Predicting Proton NMR Shielding by Alkenes Based on Ab Initio GIAO Calculations

In a strong magnetic field, nuclei located over a carbon-carbon double bond experience NMR shielding effects that are the net result of the magnetic anisotropy of the nearby double bond and various other intramolecular shielding effects. We have used GIAO, a subroutine in Gaussian 4, to calculate isotropic shielding values and to predict the proton NMR shielding increment for a simple model system: methane held in various orientations and positions over ethene. The average proton NMR shielding increments of several orientations of methane have been plotted versus the Cartesian coordinates of the methane protons relative to the center of ethene. A single empirical equation for predicting the NMR shielding experienced by protons over a carbon-carbon double bond has been developed from these data. The predictive capability of this equation has been validated by comparing the shielding increments for several alkenes calculated using our equation to the experimentally observed shielding increments. This equation predicts the NMR shielding effects more accurately than a previous model that was based on only one orientation of methane over ethene. Deshielding is predicted by this equation for protons over the center and within about 3 Å of a carbon-carbon double bond. This result is in contrast to predictions made by the long-held “shielding cone” model based on the McConnell equation found in nearly every textbook on NMR, but is consistent with experimental observations.

An Empirical Proton NMR Shielding Equation for Alkenes Based on Ab Initio Calculations

Nuclei of hydrogen atoms located over a carbon-carbon double bond in the presence of a strong magnetic field experience a perturbed magnetic field caused primarily by the magnetic anisotropy of the π bond. However, the commonly used theoretical model for predicting the shielding effect of an alkene double bond on hydrogen nuclei is sometimes inconsistent with the observed proton NMR chemical shifts in structures that have covalently bonded hydrogens located over a carbon-carbon double bond. We have used the ab initio gauge including atomic orbital (GIAO) method to calculate isotropic shielding values and to determine the proton NMR shielding increments for a simple model system: methane held at various positions over ethene. These shielding increments calculated for one proton of methane have been mapped as a function of their position in Cartesian coordinates relative to the center of ethene. A mathematical function has been fit to this three-dimensional shielding increment surface at each of four distances from the face of the ethene molecule. Additionally, a single mathematical equation has been developed for predicting the shielding caused by the carbon-carbon double bond in ethene. In contrast to the traditionally employed shielding model, our results predict deshielding for protons within 3 Å above the center of a carbon-carbon double bond, consistent with experimental observations in several molecular systems. The NMR shielding increments predicted by this equation are compared to observed shielding increments in some test alkenes.

Modeling through-space magnetic shielding over ethynyl, cyano, and nitro groups

In a strong magnetic field, covalently bonded hydrogen nuclei located over a pi bonded functional group experience magnetic shielding (or deshielding) that results from the combined effect of the magnetic anisotropy of the pi bond and various other intramolecular shielding effects. Gauge including atomic orbital (GIAO)-HF in Gaussian 98 was employed to calculate isotropic shielding values and to predict the net through-space proton NMR shielding increment for a simple model system: the proximate proton of methane held in various positions over simple molecules that contain a carbon-carbon triple bond, a carbon-nitrogen triple bond, or a nitro group. These net shielding increments of the proximate proton of methane, plotted against their Cartesian coordinates, led to the development of a single empirical equation for predicting the NMR shielding experienced by a covalently bonded proton over each group. The predictive capability of each equation has been validated by calculating shielding increments of protons over the functional group in known structures. These shielding increments are then used to adjust predicted chemical shifts for through-space shielding effects, and the adjusted values are compared to experimentally observed chemical shifts. The algorithms for predicting the shielding increment for a proton over these functional groups can be used in a spreadsheet or incorporated into software that estimates chemical shifts using additive substituent constants or a database of structures. Their use can substantially improve the accuracy of the estimated chemical shift of a proton in the vicinity of these functional groups, and thus assist in spectral assignments and in correct structure determination.

Regiospecific oxidation of substituted 1-benzyl-3,4-dihydro-isoquinolines using singlet oxygen☆

Abstract Singlet oxygen regiospecifically oxidizes 1-benzyl-3,4-dihydroisoquinoline. Mechanistic studies do not distinguish between a dioxetane or the alternative zwitterionic peroxide intermediate.

Computed NMR Shielding Effects over Fused Aromatic / Antiaromatic Hydrocarbons

Ned H. Martin *, Mathew R. Teague and Katherine H. Mills Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5932, USA; E-Mails: mattrteague@gmail.com (M.R.T.); khm0327@alum.uncw.edu (K.H.M.) * Author to whom correspondence should be addressed; E-Mail: martinn@uncw.edu. Received: 14 January 2010; in revised form: 17 March 2010 / Accepted: 19 March 2010 / Published: 22 March 2010 Abstract: Through-space isotropic NMR shielding values of a perpendicular diatomic hydrogen probe moved in a 0.5 A grid 2.5 A above several polycyclic aromatic/antiaromatic ring and aromatic/aromatic hydrocarbons were computed with Gaussian 03 at the GIAO HF/6-31G(d,p) level. Combinations of benzene fused with cyclobutadiene, with the tropylium ion, and with the cyclopentadienyl anion were investigated. Subtraction of the isolated H

Ab initio calculation of through-space magnetic shielding of linear polycyclic aromatic hydrocarbons (acenes): extent of aromaticity.

GIAO-HF within Gaussian 03 was employed to compute the NMR isotropic shielding values of a diatomic hydrogen probe above a series of acenes (linear polycyclic aromatic hydrocarbons). Subtraction of the isotropic shielding of diatomic hydrogen by itself allowed the determination of computed through-space proton NMR shielding increment surfaces for these systems. Shielding was observed above the center of each aromatic ring, but the magnitude of calculated shielding above each ring center depends on the number of fused benzenoid rings. The computed shielding increments above each ring center were correlated to other measures of extent of aromaticity, including geometric, energetic, and magnetic measurements.

Computed NMR shielding increments over unsaturated five-membered ring heterocyclic compounds as a measure of aromaticity

The GIAO-HF method within Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe at various distances (2.5 A, 3.0 A and 4.0 A) above the plane of 15 conjugated five-membered ring heterocyclic compounds: pyrrole, furan, thiophene, and phosphole and their 2- and 3-nitrogen analogs. Subtraction of the isotropic shielding value of diatomic hydrogen by itself from each of these isotropic shielding values gave the shielding increment (Deltasigma) for each probe position. Plotting Deltasigma against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment at 2.5 A above the ring center (Deltasigma(2.5)) correlated reasonably well with other established methods of assessing aromaticity, including geometric (HOMA, harmonic oscillator model of aromaticity), energetic (ASE, aromatic stabilization energy), and magnetic (NICS, nucleus-independent chemical shift) criteria.

Computed NMR shielding of phosphorus-containing conjugated five-membered ring heterocycles as a measure of aromaticity

The GIAO-HF method in Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe oriented perpendicular to the plane and moved in a square grid 2.5Å above the plane of conjugated five-membered ring heterocyclic compounds: pyrrole, furan, phosphole and thiophene and their phosphorus containing analogs. Subtraction of the calculated isotropic shielding value of diatomic hydrogen from each of these isotropic shielding values gave the shielding increment (Δσ) for each probe position. Plotting this value against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment 2.5Å above the heterocyclic ring center correlated reasonably well with the other published methods of assessing aromaticity of these systems ASE (aromatic stabilization energy), Λ (exaltation of magnetic susceptibility) and NICS(1) (nucleus-independent chemical shift) values, both of which are magnetic criteria. The magnitude of the shielding increment also correlated with the number of phosphorus atoms in the ring. Greatest shielding (5.5ppm) was observed 2.5Å above 2,3,4,5-tetraphosphathiophene.

Computed NMR shielding increments over benzo-analogs of unsaturated five-membered ring heterocyclic compounds as a measure of aromaticity.

The GIAO-HF method in Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe moved in a square grid 2.5A above the plane of 15 benzo-fused analogs of conjugated five-membered ring heterocyclic compounds: pyrrole, furan, thiophene, and phosphole and their 2- and 3-nitrogen analogs. Subtraction of the calculated isotropic shielding value of diatomic hydrogen from each of these isotropic shielding values gave the shielding increment (Delta sigma) for each probe position. Plotting this value against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment 2.5A above the heterocyclic ring center correlated reasonably well with the only other published method of assessing aromaticity of these systems ASE (aromatic stabilization energy) and with our calculated NICS (nucleus-independent chemical shift) values, another magnetic criterion. The magnitude of the shielding increment measured over the benzene ring midpoint did not correlate well with other measures of aromaticity, however.