Of course, without the actual specifics... you know, like the make and model of your laptop so we can confirm that it actually does have an 802.11n card in it, all of this is speculation.
For example, if you are using an Intel Centrino based product, you are limited to the B/G for an ad hoc network if you want to use the 2.4ghz band, and A if you want to use the 5ghz band.
This does not mean that an AD Hoc Network can't be set up. For example, if you were using a Linksys Rangeplus Wireless-N USB apdater v2.0, in the hardware properties dialog you would find an option to enable ad hoc 802.11n support. It is disabled by default. You can check the hardware properties dialog of your network adapter to see if you also have this option, and if it is disabled by default. Enabling it may allow you to increase connection speeds.
Of course, now you might be seeing why including more information is helpful.