No abstract

An accurate and efficient procedure was developed for performing C NMR chemical shift calculations employing density functional theory with the gauge invariant atomic orbitals (DFT-GIAO). Benchmarking analysis was carried out, incorporating several density functionals and basis sets commonly used for prediction ofC NMR chemical shifts, from which the B3LYP/cc-pVDZ level of theory was found to provide accurate results at low computational cost. Statistical analyses from a large data set of C NMR chemical shifts in DMSO are presented with TMS as the calculated reference and with empirical scaling parameters obtained from a linear regression analysis. Systematic errors were observed locally for key functional groups and carbon types, and correction factors were determined. The application of this process and associated correction factors enabled assignment of the correct structures of therapeutically relevant compounds in cases where experimental data yielded inconclusive or ambiguous results. Overall, the use of B3LYP/cc-pVDZ with linear scaling and correction terms affords a powerful and efficient tool for structure elucidation.

1'0DC129 Vlcm Torr %TIC 3 0 30 (155.,,,,,'" 0 1 0 3 Torr % T I C 2 0 '0 4 0 60 8 3 803 2 0 ETHYL BUTYRATE 2 o L _ _ . d 1 0 43 60 8 0 1 0 3 1 2 0 "ROPYL PRCP OluA-E Flgure 3.Water CI spectra of ethyl butyrate and propyl propionate obtained in the drift-tube CI source with water reagent gas at 110 OC and 0.103 Torr. The drift field strength was 129 Vkm-Torr, sufficient to produce substantial amounts of fragmentation characteristic of these two structural isomers preferential solvation of the product ion favors formation of RCOOH2+-OH2 from (RCOOR')H+.OH, over formation of RCOOH2+ from (RCOOH')H+. Inasmuch as the origins and analytical utility of all of the various ions observed in the CI spectra of esters have been discussed extensively in previous work (6,8,14), there is no ease of identifying esters from their water CI spectra at high E / P in the drift tube source, Figure 3 compares the spectra of ethyl butyrate and propyl propionate at 129 V/cm-Torr. The MH+ ions at m / e 117 are present for both esters, but none of the other peaks are identical. The fragment ions in ethyl butyrate are R+ ( m / e 43), RCO' ( m / e 711, RCOOH2+ ( m / e 89), and RCOOH2+.0H2 ( m / e 107), while the fragment ions \ need to repeat that information here. As an example of the The use of "in-beam" sample introduction to obtain chemical ionization (CI) mass spectra of thermally labile compounds and salts of low volatilHy is descrlbed. Samples are introduced on the surface of Teflon tubing and the effects of changing the sample position in the source are shown. The time and temperature dependence of some CI mass spectra are shown.

The work reported here has applications in neutron detection and 14C dating. In detection, one wants to measure a signal in the presence of the natural neutron background created by primary cosmic ray particles incident on the atmosphere. Neutron sources created by these cosmic rays and resulting fluxes are calculated as a function of depth in air, seawater, ground, iron, and aluminum. What is new here is the analytical generation of cosmic ray neutron sources as a function of depth in various materials. Good agreement is obtained with experimental data for both neutron spectra and fluxes in the air over ground geometry. The air/seawater geometry has the lowest cosmic ray induced interface neutron flux (3.1 × 10−3 neutrons/(cm2 s)) followed by air/ground (6.4 × 10−3), air/aluminum (2.1 × 10−2), and the highest interface value for air/iron (7.7 × 10−2 neutrons/(cm2 s)).

Field desorption mass spectrometry (FDMS) has been used to analyze three types of polyglycol oligomers-poly (ethylene glycol), poly (propylene glycol), and poly (tetrahydrofuran). Average molecular weight parameters (M" and Aiw) were determined for several low molecular weight batches of these polymers by using either protonated (MH+) or cation attachment (MNa+) ions. Despite the fact that these polymers are thermally and structurally fragile, good agreement was found between the FDMS-derived parameters and those derived via classical methods. With these results it appears that average molecular weight determinations can be made by FDMS for a large number of oligomeric systems.