I think the one you're looking for is "Brilliance, or The Necessity of Precision" by naynymic: http://community.livejournal.com/st_xi_kink/4104.html?thread=8463880#t8463880
There's also the revers - with Jim wooing an oblivious Spock by de_gnome here: http://community.livejournal.com/st_xi_kink/7586.html?thread=19547554#t19547554
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Date: 2011-02-03 07:14 am (UTC)http://community.livejournal.com/st_xi_kink/4104.html?thread=8463880#t8463880
There's also the revers - with Jim wooing an oblivious Spock by de_gnome here:
http://community.livejournal.com/st_xi_kink/7586.html?thread=19547554#t19547554