Bulletin Autumn‧Winter 1999

A p t f o r S t ud y i ng Trans i en t Chemi cal Systems I n a project conducted b y Profs. L i Wai Kee and So Suk Ping of the University's Department of Chemistry, ab initio calculations were emp l oyed t o study the structures and energetics of novel chemical species, i n particular, chemical systems k n o wn as 'transient reaction intermediates'. As their name suggests, the lifetime of these species is exceedingly short, u s ua l l y i n the order o f 0.000,000,000,000,1 second. Hence, experimental investigation s of t hem are extremely d i f f i cu lt an d expensive, and high-level calculations offer the least costly, and very likely, the most reliable w a y to study them. Indeed, as testified b y the project, i n the s t udy of transient intermediates , experiment and calculation frequently complement each other . Some Typi cal Results The CUH K researchers, i n collaboration w i t h their counterparts f r om I owa State University i n the US and the Universit y of Science and Technology of China, s t ud i ed ma ny no v el chemical systems using the earmarked grant of HK$502,00 0 f r om the Research Grants Council. I n the first part of the project the researchers studied unstabl e and short-lived novel chemical species. I n one representativ e study, they attempted to predict the structure of the cation GeH 7 + (a species consisting of one ge rman i um atom and seven hydroge n atoms w i t h an o v e r a l l c ha r ge of +1 ), w h i c h has n e v e r b e e n o b s e r v ed experimentally, using the analogou s species CH 7 + (one carbon atom w i t h seven hydroge n atoms w i t h a charge of +1) and SiH 7 + (one silicon a t om w i t h seven h y d r o g en atoms w i t h a charge of +1) w h i c h are k n o w n to exist. The cations, i.e., ions w i t h a positive charge , are considered analogous because ge rman i um, carbon, and silicon are elements of the same family. GeH 7 + is expected to be detected i n the foreseeable future since its analogous species have already been detected. It is recognized that the eight atoms i n GeH 7 + ma y be arranged i n numerous ways and tha t GeH 7 + w i l l adopt a structure w i t h the least electronic ene r gy s i m p l y because th e mo s t c om f o r t a b le configuration or shape for an y object is the one r equ i r i n g the least energy. The researchers' calculations indicated that this species has the structure shown i n Figure 1, similar to that of SoH 7 + bu t different f r om that of CH 7 + . Furthermore they predicted that it only takes about 5 kcal mo l - 1 of energy, slightly more than r oom temperature, to break Research News 35