Uncertainties In Experimental Values Research Articles

Determination of enthalpies of formation of energetic molecules with composite quantum chemical methods

We report gas-phase enthalpies of formation for the set of energetic molecules NTO, DADE, LLM-105, TNT, RDX, TATB, HMX, and PETN using the G2, G3, G4, and ccCA-PS3 quantum composite methods. Calculations for HMX and PETN hitherto represent the largest molecules attempted with these methods. G3 and G4 calculations are typically close to one another, with a larger difference found between these methods and ccCA-PS3. Although there is significant uncertainty in experimental values, the mean absolute deviation between the average experimental value and calculations are 12, 6, 7, and 3kcal/mol for G2, G3, G4, and ccCA-PS3, respectively.

Prediction of Nitrogen and Oxygen NMR Chemical Shifts in Organic Compounds by Density Functional Theory

Sum-over-states density functional perturbation theory (SOS-DFPT) and Hartree−Fock (HF) theory based on the “individual gauge for localized orbitals” (IGLO) scheme were used to calculate 13C, 14N/15N, and 17O NMR chemical shifts of 23 molecules. Employing the (11s7p2d/6s2p)[7s6p2d/4s2p] basis set, SOS-DFPT chemical shifts were obtained that are clearly better than HF- or UDFT-based chemical shifts, as is reflected by mean absolute deviations of 2.8 (SOS-DFPT), 7.6 (UDFT), and 5.6 ppm (HF) for 13C chemical shifts, 11.8, 22.1, and 100.5 ppm for 14N/15N chemical shifts, and 36.4, 57.2, and 45.7 ppm for 17O chemical shifts with regard to experimental values. By estimating appropriate gas phase values for 14N/15N chemical shifts from measured solvent effects, average errors of SOS-DFPT chemical shifts are reduced to 3 ppm, which is close to uncertainties in experimental values. SOS-DFPT is the method of choice for predicting reasonably accurate NMR chemical shifts at relatively low computational cost even in c...

Physical significance of employing self-consistent orbitals (SCO) with uniform long-range behavior

The long-range behavior of Hartree–Fock wave functions has been found to result in uniform orbital behavior at large r for the first row ground state elements. Numerical techniques were developed to overcome exchange blowup at large r, and generate HF orbitals without ’’wobble.’’ Magnetic susceptibilities, nuclear shielding constants, electron scattering factors, and hyperfine structure constants were calculated to investigate the physical reality of these orbitals. The uncertainties in experimental values are too great to permit definitive conclusions. Agreement, however, between calculated values and recent experimental evaluations of 〈r2〉 was found to be very good. The present results seem to be the first indication of the necessity of considering orbitals with uniform long-range behavior for the proper description of phenomena dependent on wave function values far from the nucleus.