Determination of Bond Dissociation Energies Using Electrospray Tandem Mass Spectrometry and a Derived Effective Reaction Path Length Approach

Abstract

A new approach for calculating bond dissociation energies (BDEs) from ES-MS/MS measurements has been developed. The new method features a "derived effective reaction path length" that has been applied to measure BDEs of alkali metal (Li+) adducts and halide (Cl-) adducts of monoacylglycerol, 1,2-diacylglycerol, and 1,3-diacylglycerol lipids. Also studied were lithium-bound dimers of monoacylglycerols, 1,2-diacylglycerols, and 1,3-diacylglycerols. BDEs for the adducts and dimers of the lipids were derived from collision-induced dissociation experiments using a triple quadrupole mass spectrometer with electrospray as the ionization source. Mass spectral data were used to empirically derive a single-exponential growth equation that relates product cross section to collision energy. From these single-exponential equations, a general second-order polynomial was derived using a multivariate growth curve model that enables prediction of BDEs of unknown complexes. Mass spectral results were compared to computer-generated bond dissociation energies using Becke-style three-parameter density functional theory (B3LYP, employing the Lee-Yang-Parr correlation functional), with excellent agreement between experimental and theoretical energy values. The newly developed method is general in nature and can be used for the measurement of metal or halide ionic adduct bond dissociation energies and for the measurement of bond energies of noncovalent interactions such as dimer dissociation energies. The validity of the method has been rigorously established using a triple quadrupole, but it may also be applied to other mass spectrometers that allow user control of the collision cell potential.