This is the 7197th most frequent Russian word.
вашингтоном
"Вашингтоном" is the instrumental case of "Вашингтон" (Washington), meaning "by/with Washington" in Russian.
The word 'Вашингтоном' in this sentence is an instrumental case, used to signify the means or the matter being discussed.
Мы ехали за покупками в Вашингтон и были восхищены Вашингтоном.
We went shopping to Washington and were impressed by Washington.
Here, 'Вашингтоном' is part of a construct with the verb 'интересоваться' which requires the object to be in the instrumental case.
Она интересуется Вашингтоном и хочет узнать больше о его достопримечательностях.
She is interested in Washington and wants to learn more about its attractions.
In this sentence, 'Вашингтоном' refers to George Washington as the doer of the action, represented in the instrumental case.