JQGrid Sorting - how to trigger onSortCol event

I am trying to get the onSortCol event get fired when I press a column header. Currently, when I click a column header, I can see a request going to the server but I want the onSortCol to be fired before this happens. I have pasted below the code I am using.

Am I missing anything? How do i get onSortCol to work?

jQuery("#list").jqGrid('navGrid', "#pager", { edit: false, add: false, del: false },
{onSortCol:function (index, columnIndex, sortOrder)
 {
    alert(index);
    return 'stop';
 }
});