Isomaltose (C12H22O11)with a α-(1-6)-linkage and laminaribiose (C12H22O11) with a β-(1-3)-linkage show different types of cross-ring fragmentation in mass spectra after treated by QTH (Quartz Tungsten Halogen Lamp) treatment. Unusually, the isomaltose has the unique 0.2A2 fragment which was not found in the spectrum of QTH- treated laminaribiose. In this thesis, the ab initio electronic structure calculations are executed to explore dissociation channels of neutral isomaltose and laminaribiose. The single reactants, reactive intermediates, transition states, and decomposition products of isomaltose and laminaribiose geometries and frequencies harmonic were computed via B3LYP/cc-pVTZ calculations. The reaction pathways of isomaltose and laminaribiose indicate the higher activation energy of 0.2A2 fragment in laminaribiose than that of isomaltose, which means the higher energy to release that fragment in laminaribiose than isomaltose. It appears to provide an explanation for the unique 0.2A2 fragment which was not found in the spectrum of QTH-treated for laminaribiose. The results are to be compared with and to explain the observed mass spectra.

1. Introduction 1
2. Theoretical methods 3
2.1 Density Functional Theory 4
3. Result and Discussions 5
3.1 Neutral isomaltose 5
3.2 Dissociation channels of neutral isomaltose 5
3.3 Neutral laminaribiose 11
3.4 Dissociation channels of neutral laminaribiose 11
4. Conclusion 16
5. Reference 17
Table 1 . 19
Table 2 . 29
Figure 1 . 39
Figure 2 . 47
Figure 3 . 48
Figure 4 . 49
Figure 5 . 50
Figure 6 . .51
Figure 7 . 60
Figure 8 67
Figure 9 . 75
Figure 10 76
Figure 11 . 77
Supporting information 78

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