טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGershoni Poranne Renana
SubjectAromatic Systems
DepartmentDepartment of Chemistry
Supervisor Professor Amnon Stanger
Full Thesis textFull thesis text - English Version


Abstract

Aromaticity is one of the most prevalent concepts in chemistry literature, and aromatic systems play key roles in various fields, from biology and medicine to astronomy and materials engineering. Yet, this fascinating phenomenon is still not fully understood. In this research, we study the concept of aromaticity with computational tools.

Nucleus-independent chemical shift (NICS)-based methods are very popular in the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift.

The seventeen isomers of the [4]- and [5]phenylenes are studied with current-density analysis (CDA) and by calculation of the out-of -plane contribution to nucleus-independent chemical shifts (NICSπZZ). Current-density maps for these isomeric phenylenes are typically dominated by strong paratropic ring currents in four-membered rings. The relative energies of the isomers, which differ only in the effects of differential strain and aromaticity, are computed at the B3LYP/6-311G* computational level. It is found that the CDA results correlate well among themselves and with NICSπZZ. The latter correlation is improved when the ring sum ΣNICSπZZ for each isomer is correlated to the ring-current sum ΣJ extracted from CDA. The strain-corrected relative energies of the isomers correlate linearly with ΣNICSπZZ. The seventeen isomers of the cata-condensed benzenes show comparable correlations between the magnetic parameters, but vastly different results for the relationship between magnetic and energetic criteria. These observations and the conclusions drawn from them lead to the development of a new methodology.

One of the main disadvantages of NICS is that it cannot determine the source of the field. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this research, a NICS-based method for the determination of local and global ring currents in conjugated multi-ring systems is presented. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ, but it is shown that at a height of 1.7 Å above the molecular plane NICSZZ provides the same qualitative picture. This method, which is termed the NICS-XY-Scan, gives information equivalent to that obtained with current density analysis methods, and in some cases, provides even more details.