New Quantitative Structure-Fragmentation Relationship Strategy for Chemical Structure Identification Using the Calculated Enthalpy of Formation as a Descriptor for the Fragments Produced in Electron Ionization Mass Spectrometry: A Case Study with Tetrachlorinated Biphenyls

Abstract

Differential mass spectrometry correlated with quantum chemical calculations (QCC-ΔMS) has been shown to be an efficient tool for the chemical structure identification (CSI) of isomers with similar mass spectra. For this type of analysis, we report here a new strategy based on ordering (ORD), linear correlation (LCOR) algorithms, and their coupling, to filter the most probable structures corresponding to similar mass spectra belonging to a group with dozens of isomers (e.g., tetrachlorinated biphenyls, TeCBs). This strategy quantifies and compares the values of enthalpies of formation (Δ(f)H) obtained by QCC for some isobaric ions from the electron ionization (EI)-MS mass spectra, to the corresponding relative intensities. The result of CSI is provided in the form of lists of decreasing probabilities calculated for all the position-isomeric structures using the specialized software package CSI-Diff-MS Analysis 3.1.1. The simulation of CSI with ORD, LCOR, and their coupling of six TeCBs (IUPAC no. 44, 46, 52, 66, 74, and 77) has allowed us to find the best semiempirical molecular-orbital methods for several of their common isobaric fragments. The study of algorithms and strategy for the entire group of TeCBs (42 isomers) was made with one of the optimal variants for the computation of Δ(f)H using semiempirical molecular orbital methods of HyperChem: AM1 for M(+•) and [M - 4Cl](+•) ions and RM1 for [M - Cl](+) and [M - 2Cl](+•). The analytical performance of ORD, LCOR, and their coupling resulted from the CSI simulation of an analyte of known structure, using a decreasing number of isomeric standards, s = 5, 4, 3, and 2. Compared with the results obtained by a classical library search for TeCB isomers, the novel strategies of assigning structures of isomers with very similar mass spectra based on ORD, LCOR, and their coupling were much more efficient, because they provide the correct structure at the top of the probability list. Databases used in these CSI do not contain mass spectra, as in the case of a library search, but a series of Δ(f)H values obtained by QCC. These techniques are capable of relating relative intensities to the chemical structures of analytes via Δ(f)H of ions which turns out to be a good quantitative structure-fragmentation relationship (QSFR) descriptor.

Atom Pair Contribution Method: Fast and General Procedure To Predict Molecular Formation Enthalpies

“…However, more computationally efficient methods remain of interest. They are invaluable for the automatic generation of reaction mechanisms, for experimentalists in need of quick enthalpy estimates to analyze measurements, for education purposes, or when it comes to high-throughput screening of Δ f H ° or related properties with the goal of identifying the most promising compounds with regard to a given application, especially in such fields as energetic materials, fuels, and alternative power sources. , In such cases, additivity methods are especially attractive because of their outstanding simplicity and extremely low cost. However, simple bond contribution (BC) methods exhibit clear limitations, , while the application of group contribution (GC) schemes to compounds of practical interest is often hampered by a lack of suitable parameters …”

Computed Mass-Fragmentation Energy Profiles of Some Acetalized Monosaccharides for Identification in Mass Spectrometry

“…The mass spectrum does not indicate the absolute value of ionic currents but does represent their relativity in arbitrary units. Thus, it was expected that the most useful mass energy profiles for structure searching would be those whose relativities ensured the best inverse correlation with the profiles of the corresponding ionic currents [17]. Obviously, this was established only when validating with the mass spectra of the chemical standards.…”

“…This is possible because the function does not change its value when the terms of one of the compared strings are added (subtracted) or multiplied by the same number. This function is part of a relationship that can accurately highlight the relative differences between calculated profiles and can order them according to the matched experimental energy profiles [17].…”

“…The semi-empirical methods RM1 and PM7 were chosen because they have provided good results in previous studies and are more affordable, and DFTs were chosen because they are current and considered more efficient [13][14][15][16][17]. They also offer a variety of mass energy profiles.…”

“…Some techniques that use the quantitative structure-fragmentation relationship (QSFR) can filter the candidate structures using the reverse ordering of the calculated formation enthalpies (∆ f H) of their primary ions, together with the corresponding experimental ionic currents obtained by mass spectrometry. These are, however, limited to groups of isomers with a high similarity, where n possible structures are attributed to n unknown isomers (n = 2 to 6) or they require at least two isomeric standards of the analyte [13][14][15][16][17].…”