LaTeX Creating Table with Different Last 3 Rows

[Rajini]

hello all,
in the following code you can create a 6 column table. But i want to make the last 3 rows a bit different. How to make the last 3 rows to have only 4 columns and the 4th column to have some text, which is common for all the last 3 rows.
thanks

\begin{table}[!t]
\caption{Assignments of DFT calculated wavenumbers.}\label{tab:1stas}
\vspace{3mm}
\centering
\begin{tabular}[width=1\linewidth]{lccccl}
\hline\hline
Experimental & FC from     & $e_j^2$ & DFT calculated & FC from      & Mode description\\[0ex]
wavenumber   & NIS         &         & wavenumber     & DFT          & \\[0ex]
[cm$^{-1}$]  &[N~m$^{-1}$] &         & [cm$^{-1}$]    & [N~m$^{-1}$] & \\[0ex]
\hline
\multirow{3}{*}{390.9} & \multirow{3}{*}{79.08}& \multirow{3}{*}{0.45} & 394.5 & 68.97 & Breathing mode, $A_1^{\prime}$\\
& &                                                                    & 400.1 & 82.27 & Breathing mode, $A_1^{\prime}$\\
& &                                                                    & 410.6 & 90.86 & $A_2^{\prime\prime}$\\
\hline
446.8 & 27.27 & 0.10 &&&\\
538.4 & 3.29  & 0.01 &&&\\
595.7 & 43.99 & 0.13 &&&\\
\hline\hline
\end{tabular}
\end{table}

 

[las3rjock]

http://en.wikibooks.org/wiki/LaTeX/Tables#Rows_spanning_multiple_columns

 

[Rajini]

Hi las3rjock,
But nothing on parbox is given in that link. It doesn't help me in anyway. For i want to span the 4th column of the last 3 rows as single. That means 4th column and 3, 4, and 5 row should be as a single cell. And this should not affect 1st row.