No abstract

The analysis of N-linked glycans by mass spectrometry (MS) has been characterized by low signal-to-noise ratios and high limits of detection due to their hydrophilicity and lack of basic sites able to be protonated. As a result, every step in glycan sample preparation must be thoroughly optimized in order to minimize sample loss, contamination, and analytical variability. Importantly, properties of glycans and their derivatized counterparts must be thoroughly studied in order to exploit certain characteristics for enhancing MS analysis. Herein, the effectiveness of the incorporation of a permanent charge is studied and determined to hamper glycan analysis. Also, a procedure for glycan hydrazone formation is optimized and outlined where a large number of variables were simultaneously analyzed using a fractional factorial design (FFD) in order to determine which conditions affected the reaction efficiency of the hydrazone formation reaction. Finally, the hydrophobic tagging of glycans is shown to be a viable opportunity to further increase the ion abundance of glycans in MS.

A self-consistent procedure for constructing a quasidiabatic Hamiltonian representing N(state) coupled electronic states in the vicinity of an arbitrary point in nuclear coordinate space is described. The matrix elements of the Hamiltonian are polynomials of arbitrary order. Employing a crude adiabatic basis, the coefficients of the linear terms are determined exactly using analytic gradient techniques. The remaining polynomial coefficients are determined from the normal form of a system of pseudolinear equations, which uses energy gradient and derivative coupling information obtained from reliable multireference configuration interaction wave functions. In a previous implementation energy gradient and derivative coupling information were employed to limit the number of nuclear configurations at which ab initio data were required to determine the unknown coefficients. Conversely, the key aspect of the current approach is the use of ab initio data over an extended range of nuclear configurations. The normal form of the system of pseudolinear equations is introduced here to obtain a least-squares fit to what would have been an (intractable) overcomplete set of data in the previous approach. This method provides a quasidiabatic representation that minimizes the residual derivative coupling in a least-squares sense, a means to extend the domain of accuracy of the diabatic Hamiltonian or refine its accuracy within a given domain, and a way to impose point group symmetry and hermiticity. These attributes are illustrated using the 1 (2)A(1) and 1 (2)E states of the 1-propynyl radical, CH(3)CC.

Recently, it has been proposed that ab initio calculations cannot accurately treat molecules comprised of a benzene ring with a pi-conjugated substituent, for example, benzaldehyde. Theoretical predictions of the benzaldehyde barrier to internal rotation are typically a factor of 2 too high in comparison to the experimental values of 4.67 (infared) and 4.90 (microwave) kcal mol(-1). However, both experiments use Pitzer's 1946 model to compute the reduced moment of inertia and employ the experimentally observed torsional frequency to deduce benzaldehyde's rotational barrier. When Pitzer's model is applied to a system with a nonconjugated functional group, such as phenol, the model and theoretical values are in close agreement. Therefore, we conclude the model may not account for conjugation between the substituent and the pi-system of benzene. The experimental values of the benzaldehyde rotational barrier are therefore misleading. The true rotational barrier lies closer to the theoretically extrapolated limit of 7.7 kcal mol(-1), based on coupled cluster theory.

Electronic structure theory has been applied to the naphthalene-, anthracene-, tetracene-, and pentacenebased radicals and their anions. Five different density functional methods were used to predict adiabatic electron affinities for these radicals. A consistent trend was found, suggesting that the electron affinity at a site of hydrogen removal is primarily dependent upon steric effects for polycyclic aromatic hydrocarbons. The results for the 1-naphthalenyl and 2-naphthalenyl radicals were compared to experiment, and it was found that B3LYP appears to be the most reliable functional for this type of system. For the larger systems the predicted site specific adiabatic electron affinities of the radicals are 1.51 eV (1-anthracenyl), 1.46 eV (2-anthracenyl), and 1.68 eV (9-anthracenyl); 1.61 eV (1-tetracenyl), 1.56 eV (2-tetracenyl), and 1.82 eV (12-tetracenyl); and 1.93 eV (14-pentacenyl), 2.01 eV (13-pentacenyl), 1.68 eV (1-pentacenyl), and 1.63 eV (2-pentacenyl). These electron affinities are 0.5-1.5 eV higher than those for the analogous closed-shell singlet polycyclic aromatic hydrocarbons (PAHs); i.e., EA(anthracene) ) 0.53 eV. The global minimum for each radical does not have the same hydrogen removed as the global minimum for the analogous anion. With this in mind, the global (or most preferred site) AEAs are 1.37 eV (naphthalenyl), 1.64 eV (anthracenyl), 1.81 eV (tetracenyl), and 1.97 eV (pentacenyl).