I installed synapse on my Cinnamon desktop, and everything worked with the keyboard shortcut Ctrl+Space. Then, at some point, it stopped working properly, and I had to hold down Control and hit space twice. Now it isn't working at all. Control-space seems to make the current window go out of focus, but I can't find it in the cinnamon control center. I would like to know how to disable it for use with synapse or kupfer, as I don't need this cinnamon Control-space anyway. I run cinnamon 2.0.10, in case it helps.
Control-space on Cinnamon
keyboard shortcuts;key mapping;cinnamon
One possibility is that ibus is trapping control-space.  From http://forums.linuxmint.com/viewtopic.php?f=208&t=169930#p873888, here are directions to disable trapping of that key sequence in linux mint 17 (Qiana):Right-click the keyboard icon in the upper right corner of you Cinamon UI, and click preference. This should open the ibus preference. Here you shoud see the Next input method shortcut coming first and set to Ctrl-Space. Click on the '...' button next to the field, and in the popup click 'delete' and validate (or set another shortcut if you need it).