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The identification of position aromatic isomers through CSI-Diff-ms

With CSI - Differential Mass Spectrometry Data Analysis (CSI-Diff-ms v1.2) position aromatic isomers could be identified, e.g. 3- and 4- nitrobenzophenone dimethyl acetals:

The mass spectra of these substances (notes by isomer X and isomer Y) are similar, as the command "Correlated spectrum" of CSI-Diff-ms v1.2 illustrates:

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Note: the differential spectra can be calculated with Differential Mass Spectrometry Data Analysis (Diff-ms v1.1 or above) too. This package is included into CSI-Diff-ms.

The classical "library search" didn't find the two compounds in our database because these doesn't contain the mass spectra of these substances recently obtained. The search algorithms are showing good results as soon as the 3- and 4- nitrobenzophenone dimethyl acetals spectra are introduced in the database. The algorithms provide similarity differences about 15-40% at the comparison of the two structures. However it's exciting and important to know if only MS can establish the 3 or 4 position of nitro group on the benzene ring without using etalons and without usage of the "orto effect" for this. A total 80.8% similarity indicates a sufficient difference for CSI-Diff-ms permitting to establish those two structures.

With CSI-Diff-ms v1.2 (or using Diff-ms v1.1 or above) were imported the spectra, calculated percentage ionic current (IC) and the ratio of spectra (Pms), values illustrated in next snapshot and table 1.

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Few more important picks from spectra corresponding to some ions with known structure (col. 1 and 2, Table 1) was selected. The spectrum differentiation was made by CSI-Diff-ms v1.2 (or using Diff-ms v1.1 or above) and differential mass spectra are illustrated in next snapshot:

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The heat of formation by molecule (ΔH) of cations resulted through fragmentation of two positions isomers, colons 6 and 7, were calculated by a Computational Quantum Chemistry method.

Table 1:

where IC is percentage ionic current; *average values; Pi is ratio of IC; ΔH is the heat of formation by molecule.

The CSI-Diff-ms platform and technology provided by BET2 Software, based on thermodynamic principles application in the case of similar fragmentation of isomer molecules, has the ability to determine without any doubts the structures and provides the results:

  • isomer X : 3-nitrobenzophenone dimethyl acetal
  • isomer Y : 4-nitrobenzophenone dimethyl acetal

This result was confirmed by NMR analysis: E. Sisu, I. Sisu, N. Dinca, C. Csunderlik, I. Oprean, V. Rusu, M. Mracec, Nitrobenzophenones and their derivatives. , 3.1 New acetals of mono and dinitrobenzophenones, Rev. Roum. Chim, 2002, 47(3-4), 339-342.

The CSI-Diff-ms software has the major advantage that permits a fast data analysis and it can be used even by non experts due to the simple steps that are easy to achieve:

Step 1: The input of similar mass spectra of the desired isomers (previous snapshots).

Step 2: The input of other possible isomers by using the command Select Ion Database Record of CSI-Diff-ms. In this way the thermodynamic and kinetic data of principal ions and fragmentations, respectively are loaded automatically:

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Step 3: The selection of thermodynamic and kinetic data set by choosing the desired/proper quantum chemical method, command Identify:

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Step 4: The establishment of the structure by using Calculate Probability command:

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The CSI-Diff-ms platform and technology provided by BET2 Software has the ability to determine without any doubts the structure of some position aromatic isomers.