) 3 + . ▁For ▁Cu ( CO ) 4 + Ar ▁the ▁peak ▁is ▁at ▁ 2 1 9 8   cm − 1 ▁compared ▁to ▁ 2 1 9 3 ▁for ▁Cu ( CO ) 4 + . ▁For ▁Cu ( CO ) 2 + Ar ▁the ▁peak ▁is ▁at ▁ 2 2 2 1   cm − 1 ▁compared ▁to ▁ 2 2 1 8 . 3 ▁for ▁arg on ▁free , ▁and ▁for ▁Cu CO + Ar ▁the ▁peak ▁is ▁at ▁ 2 2 1 6   cm − 1 ▁consider ably ▁different ▁to ▁ 2 2 4 0 . 6   cm − 1 ▁for ▁Cu CO + . ▁Comput ation ally ▁predicted ▁shapes ▁for ▁these ▁mole cular ▁ ions ▁are ▁linear ▁for ▁Cu CO + Ar , ▁slightly ▁bent ▁T ▁sh aped ▁for ▁Cu ( CO ) 2 + Ar ▁and ▁a ▁trig onal ▁py ram id ▁with ▁arg on ▁at ▁the ▁top ▁and ▁a ▁flat ▁star ▁like ▁co pper ▁tr ic ar b ony l ▁forming ▁the ▁base . ▁ ▁I ons ▁studied ▁by ▁arg on ▁tag ging ▁include ▁the ▁hyd r ated ▁pro ton ▁H +( H 2 O ) n Ar ▁with ▁n = 2 ▁to ▁ 5 , ▁hyd r ated ▁ 1 8 - c rown - 6 ▁ ether ▁alk ali ▁metal ▁ ions , ▁hyd r ated ▁alk ali ▁metal ▁ ions , ▁transition ▁metal ▁ac et yl ene ▁complex es , ▁pro ton ated ▁eth yl ene , ▁and