COMPUTER PROJECT 7-4 Pyridine

1 position for one earbon atom. The parameters hr and for modifying a and p are based on eleetronegativity differenees from earbon but are not paitieularly transferable to other problems. Transferability failure is one of the reasons that HMO methods have been largely abandoned in favor of methods deseribed in Chapters 8—10. A thorough understanding of HMO methods is, however, a useful stepping stone to researeh-level ab initio and semiempirieal moleeular orbital ealeulations. 

Procedure. Subtraet xl from the input matrix above. Load the resulting upper semimatrix into MOBAS. The first element is 1,1,0.5,0. Reeall that MOBAS requires 600 of only the nonzero elements in the upper semimatrix. Obtain the eigenvalues and eigenveetors. 

Repeat the proeedure using HMO. HMO requires entry of the entire lower semimatrix, ineluding the diagonal and all zero elements. Beeause the matrix element format is II, only one symbol ean be entered for eaeh element. The numbers 0.5 and 1.2 eannot be entered in this format instead enter 1, whieh will be modified later. The initial unmodified input for pyridine is the same as that for benzene, 010010001000010100010 henee, we ean make a trial run on benzene to see if everything is working properly. 

Run benzene using HMO. Write out the full bond order matr ix, enter ing zero for any element off the tridiagonal. What is the bond order of benzene Is there any Kekule-type alternation in this model  

To prepare the input matrix for pyridine, respond to the prompt asking how many elements should be modified with 03. Follow this with 01010.5 to ehange the 

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